I came here with the same problem. WordPress 2.3.3 users cannot post comments. FYI, my administration user (me!) cannot post comments either. I have tried the default WordPress theme as well as other themes. I was hoping this thread had a solution but am disappointed to find it does not.
I thought perhaps the host provided upgrade to 2.3.3 from the older WordPress was the problem. So I completely wiped out my /blog/ directory on the host and recreated it from a fresh WordPress 2.3.3 download. No joy. I checked the MySQL database with the host provided phpMyAdmin and all appears to be in order.