I was having exactly the same problem until I found this post. I *thought* I had switched to PHP5, but when I double checked (prompted by this thread) my hosting service PHP was still set to PHP4. Doh!

My Stats:
PHP 5.2.6
CB 1.2.2
Joomla 1.5.18

Also, I had tried switching to the default & other Joomla Templates - all of them had the same problem.

It took me far, far too long to find this. To save others some time, I'm adding some helpful SEF search phrases to this thread:

Profile Page Menu displays as a list instead of a tab

Community Builder Menu does not display correctly or properly

Community Builder Profile Menus not working, tabs not appearing

Thanks All!