Ok, my first post here so go easy on me if this sounds messed up.

Let me know if everything seems like it would work. As posted prior, Amazon.com has the T610 for -$200 after rebates [amazon.com]. Order 5 through them for $500. Activate and call T-mobile and get them switched over to the $49.95 FamilyTime Plan. Get your $1500 rebates. Comes out to $11.25/mo.
Breakdown:

T610 x 5 = $500
Activation 5 X $35 = $175
Total upfront = $675

Rebates = $1500

$1500 - $675 = $825 cash back

FamilyTime Plan = $50 (2 lines) + $10 (line) + $10 (line) + $10 (line) = $80
$80 x 12 mo. = $960

$960 - $825 = $135
$135/12 = $11.25/mo. + 5 Free Phones

The crazy part of this deal is that every time you add an additional T610 and line, you actually make money. Each phone you add costs you only $155/year ($120 for service, $35 for activation), but you get $200 back so you actually make $45.

2 lines : $270 or $22.50/mo.
3 lines : $225 or $18.75/mo.
4 lines : $180 or $15/mo.
5 lines : $135 or $11.25/mo.

If I'm missing something here please feel free to shoot holes in my post.