While Josh's assertions seem definite, I think they are wrong. My final assessments (as of now) are the following -- please correct where / if I am mistaken.
- 3G (or 4G) will not be possible on either the ST18a or ST18i. It requires 1700MHz and 2100MHz, and then T-Mobile needs to allow phone compatibility. (Ref: hxxp://en.wikipedia.org/wiki/T-Mobile_USA#3G_upgrade)
- If the phone is allowed to roam onto AT&T (this is tower-to-tower dependent), it will only get 2G, even if the tower is capable of 3G on non-1700/2100 frequencies. The restriction on speed is based on the roaming agreement between T-Mobile and AT&T.
- The last (and possibly only) Xperia that had 1700MHz was the X10. I do not know if that phone was able to get 3G on T-Mobile.
Some speculation for future -- if/when AT&T acquires T-Mobile bandwidth allocation over the various frequencies may be modified. Maybe the only good news is for T-Mobile customers is for those who buy the Xperia Arc ST18a. (Ref: hxxp://en.wikipedia.org/wiki/Merger_of_AT%26T_and_T-Mobile_USA#Frequencies)