I've tried the same idea, and never got it exactly with .htaccess ( so I handled it with PHP )
The biggest problem I found, was that it Visibly redirected ( ie: the addy in the address bar changed ) instead of doing it 'silently / invisibly'
Anyways, I think you may have to use MODREWRITE in the long run, here is the Apache page for it:
http://httpd.apache.org/docs/misc/rewriteguide.html
The basic idea will be
1) it checks the domain name ( HTTP_HOST )
2) it redirects to the subdir.
I hope this helps start you off, but this topic could really use more expansion on it by someone with some modrewrite experience!
Good luck!