Re: Base theme?
Oh, and to answer your question, yes, there’s a base theme. You can create a child theme to get started straight away.
What I suggest you do is on a test site is install the current trunk version of BP 1.2 (https://trac.buddypress.org/changeset/2400/trunk?old_path=%2F&format=zip or if you know how to use SVN, https://svn.buddypress.org/trunk/). The new theme, as seen on http://testbp.org/ is called bp-default and you will find it in /buddypress/bp-themes/ however you do NOT need to move this to wp-content/themes/.
You can just build your new child theme as usual and put it in wp-content/themes/ and Bob’s your uncle