Thanks!
I just found this, and was thinking it was a no-go?
The end result is this (unless Verizon and Sprint change their policy):
Verizon iPad: Can be used with Verizon, AT&T, and T-Mobile
Sprint iPad: Can be used with Sprint, AT&T, and T-Mobile
AT&T iPad: Can be used with AT&T and T-Mobile
T-Mobile iPad: Can be used with T-Mobile and AT&T