Why update? If it aint broken dont fix it.
that's a terrible idea. WP needs to be updated. Some updates include vital security fixes
Anyway, @Alexpierce, updating WP does not affect your theme. As long as it's not the default theme you updated (and actually, moving to WP3.0 won't affect that either) I assume all of your php and css modifications were done to a theme?
If you modified core WP files, that's a bad idea from the start. Those changes would be lost. Core WP files, and the default themes that come with WP shouldn't be edited. However, on an upgrade, non WP default themes and plugins aren't touched. Well coded themes that worked in WP2.9.2 should have no problems with WP3.0
As always......backups are important!