lsof exits with status 1 and no output on Snow Leopard when selecting either a Dropbox process group or an httpd process group. (I should get a download code for Lion in the next few days. I wonder if that works better.)
Anyway, the script mentioned previously which shows file descriptors by process group is available here as pgfiles.py.
Later
The problem symptom didn’t change after installing Lion (lsof 4.84 built by Apple a couple of months ago), but the workaround of supplying lsof with a list of the pids in the group of interest appears to work fine.
The fix to lsof was trivial. I sent it to the author, so hopefully it will be in the next release.