I have observed the following behaviour with spawn-fcgi...
http://lists.edgewall.com/archive/trac/2006-August/009484.html
It seems that spawn-fcgi processes are restricted to the user's default group (or the group specified with -g or both - not sure) but in any case, the user's other groups are totally ignored. In the above example, the trac user is a member or the subversion group but a permissiod denied error is received when trying to access the Subversion repository, despite the correct permissions being set.
I have a feeling this might be related to bug #78. By the look of the changeset, the change was only applied to server.c and not spawn-fcgi.c as well.