There seems to be two ways to do it. The thread I linked above has the first way, which I haven't tried.
Some folks have had luck with this command if your disk is attached to an AEBS:
defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1
The second way is more intricate, but works.
Mount your NAS Share using AFP, have the mount be a temporary name.
You need either to have an external drive formatted as HFS or you need to create a very small partition on your internal drive and call it something like MPBackup.
Start Time Machine and choose this small disk partition.
Stop the Time Machine from doing any backup.
Time machine should have created two files with a "." notation (meaning its hidden from finder and terminal "ls" commands.
Go into the terminal application and type
This will give you all the mounted disks including your share volume from the NAS. You should see MPBackup (or whatever name you used) and the temp name that you chose for your NAS drive.
You should see two files starting with a "."
You need to copy these two files to the NAS share. You can do that by typing
cp -p /Volumes/MPBackup/.* /Volumes/<the NAS mount name>
You then need to type in:
This will unmount (eject) your MPBackup partition.
You then need to rename the NAS Share MPBackup and re-mount it.
At this point, restart Time Machine and it should see the disk.
Make sure you have sleep set to never (system preferences->energy saver) so the machine doesn't go to sleep during a backup.
After the first backup is completed, when you eject the NAS Share or your shutdown your machine, Time Machine won't see it again until you re-mount the drive correctly.
Time machine creates a file called <whatever>.sparsebundle or something to that effect.
If you double click that file in the finder, it will mount the drive and you will see a disk show up on your desktop.
Start Time Machine and it will then see the disk and start doing the incremental backups...
I'm writing this from memory, since they don't have Macs here at work. The steps are there, as you can see, its not straight forward. The file names change, so I can't give you specifics, but the above should work. I'll try and answer any questions you might have about it.
I have this working on my ReadyNAS NV+ and I did a 300GB backup over Wireless G. It took about 3 days to complete the initial backup. I came home and mounted the Drive like I did above and it did the incremental backup. Next stop is to try and restore something just to make sure it works.