I ran into some problems with WP-generated rules in 1.5. The rules they generated fell victim to Apache 1.3's "greedy" mod_rewrite regex engine. I wrote a plugin to get around this problem, and it seems that the plugin is borked on upgrade to 2.0.
Just to test it out, I stalled WP2.0 in a test environment. It seems that the rewrites were taken out of the .htaccess altogether, and are now done internally. So, I'm guessing the plugin isn't necessary any longer, as the potentially-greedy rules are now handled by php instead of mod_rewrite. Is that correct, AFAYK?
Also, is there any way to tell WP to use the "old style" rule-generation? I had tweaked that plugin to add some functionality to my URIs, and don't want to have to re-learn a whole new system if it's not really necessary.