With Windows 7 due by October and looking good, and SP1 bringing Vista up to the same performance as XP SP2 (that's what Microsoft claims its Customer Experience Improvement program telemetry shows), who's going to want Vista Service Pack 2?
Actually, even Microsoft says this is mainly for Windows Server 2008 (which shares much of the same code as Vista) and for putting on new PCs.
Cuirrently only available to MSDN and TechNet subscribers, SP2 will be available as an optional install from Windows Update and even when it moves to be an automatic update ("after some time", as Microsoft puts it), you'll get a chance to decline it.