It looks like people actually read this blog, as my observation below about June 2006 entries prompted a comment on one of them. David challenged some of what I said, and he and I may continue the discussion on facebook (the blog gets carried over) but I'll put any main entries here as they are accessible to more people.
As I said back in June '06, I'm not a 'young earth' creationist and never have been. Although I grew up in an evangelical church, where the authority of the Bible was taken very seriously (my vicar had been a UCCF travelling secretary in his younger days), a literal reading of Genesis 1-3 was not taught from the pulpit. That's important to say, because I suspect our upbringing affects our ongoing theological views rather mor than we think. Someone brought up in a church where creationism predominates is going to build it into their faith, whereas for me it would require a major change of world-view. So, the first reason I'm not a creationist is: I wasn't brought up to be one in my Christian development!
From what I have looked at in the heated exchanges that this issue produces, there is a lot of science (and pseudo-science) thrown around about geological information, species development and so on. I could swap information with people all day and get nowhere, so I won't. I think the more relevant point to make on the scientfic level is this: the assumptions which form the basis of all the science and technology we use and benefit from are the same ones that lead scientists to assume an 'old' universe. You can't separate out questioning the science that the dating of the universe is based on from the science that explains your mobile phone, provides the medical technology in your hospital or keeps the lights on. There may be missing dinosaurs, odd geological levels or inaccuracies in C14 dating, but none of this is enough to discredit the scientific case.
The key question which follows on from this, however is why would the universe fail to tell the truth about itself if it has an ultimately Divine origin - i.e. an origin in Truth. Can we seriously picture God setting up a universe with stars billions of light years from earth, along with light beams extending all the way back from these distant objects to earth to make it look like the light has travelled all that way (and hence for billions of years) when in fact it was all "switched on" only a few thousand years ago? What motive would a God of truth and love have in creating such a deception? Isn't it more coherent to believe that our ability to understand the world scientifically is, in fact, evidence that a rational God gave us the ability to undertake rational exploration of a universe that is also rational?
My final reason for rejecting a creationist proposal is that it misunderstands the nature of the text of Genesis. From what I have seen, the vocal Biblical literalists tend to regard the dating of the Universe as the key content of the text, which does it a great injustice. Genesis 1-3 are addressing theological issues about origins, and the relationship of humanity with God, with each other and with the world. Genesis introduces stewardship, sexuality, family, as well as explaining conflict, jealousy and alienation from God and each other. Scholars more qualified than me say that Genesis systematically contradicts the creation myths of neighbouring cultures, which show gods with mixed motives, and ones which are not fully in control of events. Perhaps more attention to the theology of Genesis, rather than its chronology, might mean we all gain much more from it.