I was talking to a chap from a smallish software company last week who, after the latest 'we're going to chop XP' date announcement from MS, got on the blower to Dell (who provide all their computer hardware) and sought some reassurance, in strongly worded terms, that they would continue to support XP (pre-installed on new hardware and support services).
He got the assurances that this would be forthcoming until at least 2010, as there's a huge amount of demand for it. Of course, this is for business clients but still, shows if you shout loud enough you get.
According to Ballmer MS are aiming to get Windows 7 (the Vista replacement) out the door by the end of 2009 now; I'm not surprised they wish to move on from this episode as soon as practical and am quite sure you'll be able to get XP until Windows 7 is bedded in - I suspect a large proportion of people upgrading to that version will be doing so directly from XP.