It's nice to see someone expressing my thoughts so well. David Heddle has a good post on why the insistence on what too many people in Reformed circles call "literal six days" is just silly. (Side note: for why I think this phrase is inappropriate, see my comments on David's blog. There's more on that in the extended entry below, but I suppose an explanation of my choice of words is important here if you just want to see a briefer picture and then continue reading.)
Quick summary (from the post, the comments, and the follow-up post):
1. Reformed thought generally frowns on what is often called over-literalness (though I would question that term) in other places.
2. Some of these people take it so far that they would have to exclude revered church fathers and Francis Shaeffer from being deacons.
3. It's a "misguided attempt to combat evolution" but not necessary and relatively modern as a plank of legalism.
4. Old-earth views don't necessarily (or even usually) deny inerrantism but are too often treated as if they do.
5. "Regarding the literality of Genesis: Perhaps the most important verse in Genesis is the first Messianic prophecy of Gen. 3:15. That critical verse, as we all know, was not fulfilled literally. Christ defeated Satan on the cross, but He did not literally crush Satan's head nor did Satan strike His heel." (Though, again, I would say that within the account of the prophecy the terms are being used literally -- i.e. it really is a picture of one person crushing someone else's head, but the prophecy itself isn't trying to describe physical events but spiritual realities.)
6. Then he gives some reasons not to bother wasting your time with the so-called creation science sites, which I won't bother wasting my own time (and yours if you, like me, don't need further reason to distrust them) by repeating here. If you're interested in his reasons, read his post.
I'm largely in agreement with all that. I do want to give some more depth (than my above-linked comments gave) to my big pet peeve with virtually everyone who comments on this issue.
What follows is a large expansion on my comments on David's site:
Some young-earthers object to old-earthers by saying that the exegesis shows that the word 'day' (or really 'yom') refers to literal days. Some old-earthers respond by saying that the 70 weeks in Daniel are not literal weeks. David in fact repeats this point. My problem with this is that there's no such thing as a literal day or a literal week. It's a category mistake to apply that term to something other than a unit of language. It's similar to my former roommate who used to say "I'm going to go hit the proverbial sack". Is he hitting a proverbial sack? There isn't a proverbial sack to hit. What he's doing is, proverbially or metaphorically speaking, hitting the sack. 'To hit the sack' is a metaphor, not a literal statement. It's the statement that isn't literal, not the sack.
Now on the 70 weeks of Daniel, I think you can say that 'week' is being used in some non-literal sense, since the passage itself says how it's using it. They're weeks of years, so 'week' means a sevenfold collection in general rather than a sevenfold collection of days. That is indeed a non-literal use. However, the passage tells us that this is how it's used. That's not true in Genesis 1. My conclusion, therefore, is that trying to read 'day' in Genesis as some finite amount of time longer than a day but roughly equal for each day" (i.e. the day-age view) is not the best way to view the passage. Within the narrative of Genesis 1, the term 'day' does indeed seem to be used literally to refer to 24-hour periods. That's a battle the old-earthers shouldn't need to fight, however, so I want to suggest that they don't bother. So we'll go ahead and grant that on the level of exegesis the word 'yom' is being used literally within the account.
Why is this not a problem for old-earthers? Well, 'seed' in the parable of the sower is literally talking about seeds, on the level of exegesis. Hermeneutics then lets us see what those literal seeds in the story mean -- the gospel. In the story, though, they're being used literally, but it's not really talking about seeds but what the seed in the story signifies. The story itself is non-historical, being a parable. The sentences within the story are being used literally within the story. The words are generally being used literally within the sentences. Compare when the Jews called Gentiles dogs. That unit of speech is non-literal. It's a metaphor for saying they display doglike behavior. If they were to tell a fictional story about dogs displaying Gentile-like behavior and then said "and that's what the Gentiles are like", then the term 'dog' would be used literally and the whole story then applied non-literally to Gentiles.
Within the first creation account, 'days' literally refers to days. The question is what that overall account is doing. Is it being used to illustrate spiritual principles and theological claims distinguishing God's creation from pagan creation stories, or is it a chronological, scientific account about 24-hour periods in history? I think the account, on the face of it, doesn't require the latter at all, though young-earthers do seem to think you can't be an inerrantist and take it otherwise. Old-earthers say it's the former. What I'm saying here is that the exegesis is the same on both views, but what it means in terms of what it's really saying is different. So the word for day is being used literally even on the old-earth view. It's just that the whole account is being applied to a period of time much, much longer. It's not a chronological account, and it's not describing events in a straightforward historical way, as we moderns would prefer. It's making theological claims distinguishing between the biblical view of God and the contemporary pagan creation accounts. It doesn't pretend to be ordering things the way it would in a chronologically scientific account. The poetic features most commentators notice signal that anyway.
A nice readable discussion of such literary matters is in Bruce Waltke's 2001 commentary on Genesis. The first volume of Gordon Wenham's two-volume Word commentary (1987) says similar things but isn't as clear or readable, hasn't dealt with more recent work on the topic, and, actually spends less time on it than Waltke's much shorter work. I don't remember if they use 'literally' the way I've suggested it should be used, but the substance of their comments seems right to me. Most of the science-focused stuff on this seems to miss a lot of the literary issues these commentators notice.