The only supported destinations for Time Machine are:
1. A directly attached internal or external drive/partition.
2. A drive directly connected to another mac running 10.5 on the same local network.
3. A Time Capsule
4. A drive connected to an Airport Extreme base station should work, but is not supported officially.
Anything else is unsupported by Apple, although there are some "hacks" that claim to make NAS drives work:
http://www.macosxhints.com/article.php?story=20080420211034137&query=time%2Bmach ine%2BNAS
Also, HP recently introduced a piece of hardware that can be used (starts at about $700, I think)
http://mefeedia.com/entry/macmost-now-186-hp-mediasmart-server/13439036