Well,
Once SP3 finally comes out, XP users won't have long left before support for it runs out. Shame really, XP is a good OS, and I don't know anyone who prefers Vista over it. I only run Windows in a virtual machine for Internet Explorer testing so it won't affect me in the slightest, I run Ubuntu Linux as my main OS (new release today, Hardy Heron (8.04)).
I reckon Microsoft are going to have some massive issues when they attempt to kill off XP later this year, most business systems aren't capable of running Vista because they don't have billions of megabytes of RAM or monster processors. I mean our PCs at school have a maximum of 512MB RAM, hte latest shipment of them only came in last September. We're still running XP at school, there's no way these systems will run Vista, because of hardware and software restrictions.
But back on topic, XP SP3 isn't going to be as big an improvment as SP2, but it should fix some security and stability issues; so it'd be a good idea to update.
-Luke.