Considering Verizon released the EVDO upgrade for the 7250 recently, it is totally possible that Cingular might eventually release a UMTS upgrade for the 8700.
It's an interesting concept -- not a new concept though -- include an over-featuered chipset that contains features not yet beta-tested or enabled, and release a firmware upgrade to utilize the extra features.
As far as I know, the 7290 chips do not include EDGE or UMTS capability. However, the specifications on Intel's website for the chip being used in the 8700, includes support for UMTS.
That doesn't mean there WILL be an upgrade -- but it is an interesting observation nonteheless. If this ever gets utilized, this would potentially blow away EVDO in speeds.
Don't forget how long it took before the 7250 officially got the EVDO upgrade.