This bug has been reported on Debian (http://bugs.debian.org/678402):
I tried
$ xpra start :13 $ DISPLAY=:13 urxvt & $ xpra attach :13
I get a broken urxvt because X startup scripts are not run and, among other things, .Xresources are not loaded into the Xvfb X server.
I wonder how IMs are supposed to work in absence of X startup scripts, too.
This makes xpra environment pretty much unusable.
It is not the role of xpra to deal with starting up the full X environment, that is what tools like winswitch are for.
If you must do it with xpra, you can always point --xvfb
to a script of your choosing that starts whatever you require (dbus, etc).
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/154