I think I'm having the same problem. I've tried setting it to use the Joomla registration and Joomla login, then SMF registration and SMF login, and every other combination (I don't have CB). However, whenever I try to log in with the Joomla login, it simply takes me back to the main page, and the login module doesn't say that I'm logged in (it still shows the "username" and "password" text boxes). When I try to log in via SMF, it takes me back to my site's homepage, and I'm still not logged in on either Joomla or SMF. When I clicked "verify" to sync the users during the JSMF install, I clicked the one to sync the real names.
Joomla: 1.0.13
SMF: 1.1.3
JSMF: 2.0.2
*EDIT: Looks like our answer might be here:
http://www.joomlahacks.com/component/option,com_smf/Itemid,94/topic,4216.0*EDIT #2:
YYYYYYYYEEEEEEEEEEEESSSSSSSSSSSSSSSSSSSS!!!!!!!!!!!!!!!!!!