It's possible, and i'm using it at this time !
I could have tried o make a new sparsebundle with the linkstation web admin, but I don't really know what it does, so i've done it myself.
Explanations can be found here :
http://appleclinic.wordpress.com/2008/10/30/time-machine-on-nas/
The trick is just to create a named sparsebundle file and copy it into the backup directory on your NAS.
One more thing, if you want to use Airport (yes, I know, but I don't want to plug my laptop), use the ETHERNET Card MAC Adress, not the Airport's one.
I've tried it on a macbook connected via Airport to my local network and found that TimeMachine was trying to create a sparsebundle by itself without using the one I created, and failed on connecting to the time machine volume. This file was named with the Mac Adresse corresponding to the Ethernet Card, and not the Airport One... That's strange...
So i simply renamed my sparsebundle file with the same name as the one timeMachine was trying to use, and it works...