Rumoured for Leopard, Bluetooth PAN is now in 10.4.9. I've just set up my W810i with my PowerBook, and I have a new network port: Ethernet Adaptor (en2), which is configured with a default name of Bluetooth PAN.
Note that it's now easier to connect to the internet via the Bluetooth menu using "Join Network on [your device]".
[robg adds: I can't confirm this one here, but would appreciate any confirmations -- it sounds quite useful for those with capable Bluetooth devices.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070327142723231