I tried to tether to my Bold 9780 from my PlayBook using the default T-Mobile tethering Profile on the PlayBook. I got a T-mobile screen saying I needed to sign up for a Hotspot plan for my mobile device. I have been a long time T-Mobile subscriber so Tethering should be included.
I ended up looking at the APN as setup on my Bold and created a new PlayBook Profile for tethering using this APN:
APN: epc.tmobile.com No username or password was necessary
All worked fine the next time I tried to tether from my PlayBook to my Bold.