This question really confuses me because it is a totally moot concern.
I have been with Cingular/AT&T for over 6 years...in fact so long I lost track, so what if I have 2 more years to go....where would I go? why would I go?
It seems pretty simple to me: 1) If you want an iPhone then you switch to AT&T and keep using the iPhone...no one else (stateside) carries it and there is at least a 4 more year contractual commitment between Apple/AT&T...therefore...moot!
If one has been with AT&T for over 2 years and are still using them then what is the point of worrying about any further obligations when it all works.
If one moves from the 8gig to the 16gig means that there is a 99% chance that they will buy Generation 2 and 3 so again, it is a moot point.
If one has good clear service with AT&T and they are currently upgrading their systems and one is still using then what is the concern?
16gigers did NOT sign a new contract nor was their obligation extended. You can take the sim card out of the iPhone and put it in ANY AT&T GSM phone and it will work. You just can't take an old sim card from a cheapo and put it in the iPhone.
...and last but not least...If one spends $400-$600+ on an iPhone, why in god's name would they not want to use that phone for 18 - 24 months???
I guess I just don't get the question and never will