I had the same problem, with a pendrive and with an ipod.
After reading some info, i got that the point was in the program
"pmount" that I could not execute, even if I belong to the group
"plugdev". So the solution was making
chmod a+x /usr/bin/{p,pu}mount
Now everything works correctly, except for cdroms, which I think is
due to the presence of "/dev/hdc" in the fstab.