The problem is you're using that loaded term of "4G," which is currently completely meaningless, both for what technology is being used and what speed is being offered. At the moment, a customer can't take a "4G" phone to another "4G" carrier.

Hopefully LTE will be the standard of the future. Perhaps in the timeframe you mention, it will be.

You're right, but the time I'm ready to sell off my Nexus One, the switchover probably still won't have happened, but it's still unlikely that anyone will want to buy my phone knowing it'll only be usable for a short time.

If the merger is approved, it would be irresponsible if AT&T still allowed T-Mobile to sell ANY phones that don't work on AT&T's frequencies.