After waiting a few weeks to see if any additional issues would surface for the recent Windows XP SP3 release, I went ahead and upgraded (after backing up the hard drive, of course).
I must say this was one of the most painless service packs I've dealt with - running the install meant that it checked for a few minutes to ensure compatibility, copied over a few new files, and asked for a single reboot. Done. Programs seem to load noticeably quicker so far (and every little bit counts with these computer-memory-crashing 1DsIII RAW files 
), and I haven't run into any issues.
I think SP3 was very briefly offered as an 'Automatic Update' and then was disabled when they hit the few issues, but it's available for manual download for you intrepids 
.
