How about allowing your users to select a blog theme while they signup or create a new blog on your buddypress/wordpress mu powered site.
It will be a cool thing won’t it.
Well,here is a plugin “bpdev select blog theme at blog signup” which allows this.

Download the plugin from here http://buddydev.com/plugins/buddypress-premium-plugins/allow-users-to-select-a-blog-theme-while-signing-upcreating-a-blog-on-your-buddypress-powered-social-network/
Note:this is a premium plugin ,and you must be a member of bpdev premium(which costs only $30 for 3 months) to access this plugin and many other.

screen shots

Buddypress registration screen

How it works:
You simply activate it site wide and take the nap and It will do the rest.hehe ,well nothing of that sort,but you need to install this plugin and as a site admin and you are done.

It will list all the allowed themes(in wpmu you allow themes for user from SiteAdmin->Themes) ,to the user when he/she is signing up for a new account or is an existing user and creating a new blog.

This selected theme will be automatically get activated on the new blog ,when user account is activated(In case of signup) or the new blog is created(for existing site users).
So,what do you think,Isn’t it cool!

Go and grab the plugin form here

and do let me know ,what do you think about this

happy networking.