How I got it to work:
Installed bluetooth dun
From terminal/sudo, ran "hciconfig hci0 class 0x51020c" to change N900 to a "phone" instead of a "computer"
Paired the devices (Playbook saw N900 as a phone)
Now Internet Tethering from Playbook shows N900 as an available device.

Once they're paired, the Playbook still sees the N900 as a phone, even if you reboot either device and the N900's ID reverts. I guess it only determines capabilities/profiles during initial pairing?