I installed Apache 2.2.21. All default config except that I enabled mod_rewrite and created virtual hosts for the various domains... but the rewrite rules from my first post still worked. :/
So at this point, I can only figure that there's something else in the htaccess or in the main config that's causing a conflict. If you want, you can try to post your whole htaccess and main config, and one of us might be able to spot the issue. Or you can get your host to start logging errors (which is probably a good thing to do anyway).