Registration issues with 1.5 and certain templates have been a recurring problem. Luckily, one where the solutions have for the most part been worked out. To help you I need a little more info.
1. Are you using the most current nightly build of 1.5?
2. What template are you using? (try using Beez and see if registration works. If it does I know the fix.)
3. Are new users able to register on the Joomla portion of the site?
If you would provide a link to your site I could try to register as a new user and see what's going on. You can take a look at my site to see how it is supposed to work. I'll tell you it took some doing to get Joomla and my template to work correctly. Fireboard worked fine after I got the other two in line. My site is
http://alcoholicsanonymousonline.com Go to the Study Forum to see Fireboard and register as a new user from there. You can also register from the front page through Joomla and after activation and Login Fireboard lets you right in and does not require an additional login.