Microsoft has recently updated the Service Pack Road Map. Microsoft is committed to providing a 12 month roadmap of upcoming service packs and security rollup packages and these are describe on the Service Pack Roadmap page, which was updated late last week. The main change is that MS are delaying XP SP3 for around a year. SP3 had previously been planned for mid-2007, and is now shown as not being ready till mid 2008.
Two years between service packs is a long time (SP2 was released 2 years ago), but 4 years is a long time. The more cynical reader might argue this is one way to get users to upgrade to Vista, but that would be unfair. It's more likely that MS does not have the resources to both support Vista (and while not announced the almost certainly needed Vista SP1), complete/ship Longhorn and deal with an SP for XP.
In this regard, note the comment on the road map page: "Microsoft recommends that business customers use the above table to plan for the evaluation of new Service Packs as soon as they become available and to allow for the fastest possible transition time, in order to maintain all PCs on current service packs." Of course, is MS change a date by a year without any warning, I find it hard to see how this table can have much use except after the fact.