NOT worth it. It almost doesn't seem worth it for 2G, except they'll get 3G and GPS with their upgrade as well.
AT&T won't even begin to service 7.2 HSDPA until "later this year:
http://www.gearlog.com/2009/05/att_rolling_out_hsdpa_72_faste.php
which in AT&T terms means mid next year (when the next iPhone comes out!).
More over, not signing another contract allows me to switch to Verizon if Apple does end its contract with AT&T in 2010. WOOOT!
Seems like win-win for 3G owners who are NOT upgrading.