I'm giving my brother my 8900, but I want to keep my old grandfathered Blackberry unlimited data plan. I'll be going blackberryless for the next few months using a WinMo phone.

The problem is I can't just delete the PIN that is registered to my account. I try to delete it and it says I must enter a valid PIN. I'm going to assume it will cause something funky if my brother tries to register the handset on his BIS account while the PIN is still active on my account? I don't want to lose my grandfathered cheaper BB Unlimited data plan because I plan on getting a 9700 when I get back from this assignment. Any advice?

Correct, he can't register the PIN of the device on his account until it is released from your account.

Call and ask the Tmo to remove the PIN, but not to delete your BIS account.