It's BETA. As mentioned above no need to rush out and download it. If you did you may be reloading your system before you really want to. Let the guinea pigs test it out first and wait for the gold code to be released. That will take a few months yet. Probably first quarter next year.
The one thing for sure like seen with SP1 will be the new IE 7 included along with some more recent security fixes as most of the bugs have already been worked out of XP at this late date. When trying the beta for IE 7 that was fun removing. Likewise the beta for Vista was removed promptly since these are unfinished evaluation only type releases. But SP3 is already long overdue for XP with Vista almost ready to see it's own SP1? It seems they waited until Vista was out before releasing the finished version of IE 7.
It's probably almost entirely a collection of the hotfixes from SP2 up to now with a few new ones bundled for fun. Still it beats downloading 90 updates when you install from a normal SP2 disc.
Upto 90 now? I thought it would be a little higher like 120 at this time. When reinstalling XP a few months back some 84 were seen then! That included IE 7's finished version "finally out"! MS being slow again there.