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They will be providing full support ending sometime in 2009, and "OMG huge security hole!" patches until 2014.
So it makes perfect sense that they release a SP3 for XP.
The idea that SP3 will hurt Vista adoption is true, but if someone decides not to install Vista, and install XPSP3 instead, that doesn't hurt Microsoft a whole lot.
And before 2014 you can expect to see a Service Pack 4 or ala 2k - a large security rollup package. Edited 2008-04-29 19:56 UTC