I had the same problem, and it has continued right through WP 2.3.1 using any theme, on several domains.
Is there a limit to the number of plugins that one may activate on a blog? I ask because the error does not happen until after at least 10-15 (it varies) plugins have been activated. After that, any attempt to activate ANY other plugin (even those that have worked in the past) fails, and I see the 500 error.
I’ve checked permissions and so on (755). I’ve tried deleting and re-uploading the plugin files in question, as well. Neither measure helped.
In WP 2.3, I was able to use the clean-options plugin to safely clean up the wp_options table, then install more plugins. That isn’t helping now.