For most people with Eurasian versions of smartphones, you only choice for full 3G is using AT&T because of the frequency issue. Only AT&T uses the same frequencies at 1800MHz and 2100MHz spectra for WCDMA. The reason why many can't attain 3G speed on T-mobile USA is that they use 1700MHz and 1900MHz frequencies for WCDMA which is not supported by phones outside US. Hence your iPhone would only see 2100MHz for GSM and attain EDGE speed at maximum.
Luckily I got a T-Mobile USA version of Nexus S, so I can be different and use USD3 a day unlimited phone + data at HSPA+ speeds for the first 200MB.
In response to the following email:
http://lists.wikimedia.org/pipermail/wikimania-l/attachments/20120617/056a4b...
Best regards, Alan Lai Sent on the road