I have a Motorola V66 phone, with (locked, I assume) service from
T-Mobile.  I live in norther Eastern New Jersey.  My phone recently
started having trouble with a couple of the buttons on the keypad
(they don't work when I press them), and so I am considering either:

1) Repairing my phone
2) Replacing my V66 with a different V66 (eBay) by moving the SIM
3) Getting a different phone altogether (and move the SIM)

My inclination is to get a different phone, but don't like the idea of
paying a ton of money to own one outright, nor be married to a carrier
for a year or two due to the contract required for a "free" phone.

I'd like to just fix this phone, but in attempting to open it I
noticed that the keypad subassembly emmes to be a sealed unit, and so
the effort required to repair it is probably more than the cost to
just replace it.

Q: Can get another V66 off eBay and just move my SIM card to it?
Q: What other models of phone could I move this same SIM card to?
Q: Do I have to involve T-Mobile in this?

Your SIM should work in any unlocked GSM phone.

As much as T-Mobile will hate this answer you can just switch phones
without them knowing. Because their networks runs off of the "Sim"
card system you can transfer all of your phones information
(Connection, phone book, call list...) by just moving the cord. You
could go and buy a new phone from T-Mobile but that will cost >300 for
just a simple phone unless you sign some sort of new plan. But check
with them to see if they have any trade in deals. Also look on ebay
and find one that is new. There are quite a few of them that will be
on there. If you see one that is used it would probly be ok also.

Happy hunting
~mbenc