I have a normal SD card inserted.
It was auto-mounted on /media/usb0.
What I did was:
sudo mkdir /media/usb0/test
sudo chmod 777 /media/usb0/test
The command 'sudo -u www-data touch /media/usb0/test/test' gives the following error:
touch: cannot touch `/media/usb0/test/test': Permission denied
So I think I have the same issue as you have.
The command 'mount' tells: about this mount:
/dev/sdb1 on /media/usb0 type vfat (rw,noexec,nodev,sync,noatime)
The command 'ls -alth /media/usb0/test/' tells me:
drwxr-xr-x 2 root root 16K 2010-12-12 07:38 .
drwxr-xr-x 4 root root 16K 2010-12-12 07:38 ..
So chmod did not change permissions.
I experienced this behaviour in the past and if I remember correctly,
the thing is that vfat is designed for Windows permissions.
And linux permissions and Windows permissions quite different.
Therefore setting Linux permissions on a vfat mount (sub path) does not work.
What you can do is set linux permissions and ownership for the whole tree.
You can specify this in your mount command (not with automount, I guess).
check 'man mount' for more info.
Or you might change to a linux filesystem such as ext2
I'm not a Linux guru myselve, but I hope I helped you in the right direction.