I am agnostic as to how much Genesis 1 says in terms of facts. As I said below, from a scientific point of view, I work on the basis of naturalistic assumptions - partly because I believe that they will be shown not to stand up in the fulness of time. Hence the ID slant in what I write - I think that it will become apparent that purely naturalistic explanations of the universe will be demonstrably unfeasible.
From a Christian point of view, I don't have a problem with people like Mackay (or friends of mine) arguing for a strict six day, 6000 year timetable - though I think this causes a dichotomy between creationist science and naturalistic science which does little to help either Christianity or science. For the record, I find science that has a prior commitment to philosophical naturalism (that excludes the possibility of external intelligent agency) somewhat more unhelpful. As evidenced by Alan's comments, it is quite apparent that even within creationist evangelical Christianity (I hope that's not an unfair label, Alan), there is quite a diversity of opinions - there certainly isn't a tightly controlled Young Earth Creationist line that everybody is expected to toe.
However, it may come as a shock to some readers that I don't endorse a 6x24 hour period of creation. Why am I not convinced that the six days in Gen 1 are six 24 hour periods?
1) Because (24 hour) days are reckoned according to the setting and rising of the sun, and the sun wasn't created until day 4. Arguing that it was concealed behind cloud (or something) and only became visible later on is not what the text says, and seems to be an argument to justify a prior conclusion.
2) Because the creation account in Genesis 1 is written from the perspective of God in heaven, not from a human perspective. In studying Genesis recently, it was suggested that verses like "This is the account of the heavens and the earth when they were created" [2:4] and "This is the written account of Adam's line" [5:1] actually relate to the preceding passage, rather than what follows. This makes a lot of sense, and for example helps to explain why there are two dissimilar accounts of the creation. In any case, it isn't clear that God's timescales are the same as ours: in fact, later on in the Bible, we are explicitly told that they aren't ("a day is like a thousand years").
3)... this ties into the fact that the seventh day is different. God enters his rest on the seventh day - and remains in his rest - which Adam and Eve are created to share to begin with, but which they lose. From the perspective of this passage, there is no "eighth day". This is a highly significant theological point - see Hebrews for how this is teased out through the rest of the Bible. Which doesn't exclude the possibility that the six days were 24 hour periods, but certainly makes their actual length far less significant than what happens on the seventh day. Given that the seventh day theologically doesn't end - and is apparently not a 24 hour period - it also seems less likely that the first six days were 24 hour periods.