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.)
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.