<p>With which version(s) of VS2008/VSTS can I work with .dbproj files?</p>

You can definitely use
Team Edition for Database Professionals
.  The bad news is that it seems you
can only use this
or of course the superset version
Team Suite
but that would set you back a lot for a team.
It would be nice if the whole team could edit these files without having to fork out for these expensive extra licenses.  On the plus side the dbproj projects can be
built using Team Build