Well, I didn’t see that one coming. A silly bug in WordPress 2.7.1 that hit the streets yesterday broke indexes where you have the “ignore characters” option set, and possibly other things as well, if you were running on a server that still uses PHP4. (By the way, if you did see the problem, it’s worth checking to see if your web host gives you the option to upgrade to PHP5 — mine does.)
Anyway, v0.7.6 provides a workaround to the bug (i.e. never calls the broken function) so everything should be up and running again.
This incident does prove, however, that you should exercise caution whenever you are upgrading your blog to a just released version of WordPress. Unless you desperately need something in the new release, or it patches a severe security hole, then there is no reason why you should upgrade immediately. Avoid the pain! Give the plugin developers (and early adopters) a few days to discover and resolve issues, bugs, and incompatibilities in the latest version before you upgrade yourself.