I've read all the terms and conditions and it looks to me as if I'm eligible. The T-Mobile rebate form even instructs you in sending multiple rebate forms for different numbers ("Maximum of five rebate per name or address...")
And the Amazon one says explicitly that it's fine, so I'm going to assume that it will work. If they reject me, I'll be sure to fight it; since there's nowhere that says you can't be an existing T-mobile customer. If, in the long run, I don't get the $100 rebate, its still a good deal since I'm buying this phone to resell.
Nate
edit: And a note about >$34.99 activation: with T-mo you can change the rate plan at any time, so you can activate on the $39.99 plan and then drop down to the $19 or $29 plan after a month or two w/o contract extension.
Nate