Use BuddyPress, Keep your Theme
If you want to use BuddyPress (a massive plugin that basically adds a social networking layer to WordPress sites, think, forums, profiles, user blogs, etc) but you’ve been hesitant to try it, now is a better time than ever.
BuddyPress 1.2 now works with stand-alone WordPress installs (before you had to use MU) and you can even keep your existing theme.