Real world 3G data upload speeds tend to be all over the board, but can climb upwards of 1000 kbps. In some cities, however, that seems to be capped at 100 kbps, according to chatter on the Mac Rumors forums.
Cities that appear to be impacted by upload performance issues include New York City, Suffolk County and Long Island, central and south New Jersey, Boston, Orlando, Seattle, Philadelphia, Columbus, Cleveland, west Houston, Phoenix, Denver and Northern Colorado, Minneapolis, Saint Paul and other parts of Minnesota, Detroit, Cincinnati, Baltimore, Salt Lake City, Las Vegas, Kansas City, and Fairfax.
TMO’s tests indicate that in the Denver metro area, 3G upload speeds on an iPhone 4 and iPhone 3GS don’t top 60 kbps, but tests in the San Francisco bay area sat around 815 kbps and 819 kbps. TMO’s tests in New Hampshire showed download speeds up to 859, although other tests were just over 100 kbps.
With 3G upload speed limits showing substantially slower in only some cities instead of nation-wide, it seems likely that AT&T is suffering from regional issues with its wireless network instead of intentionally limiting performance. The company is promising a statement on the issue, so hopefully an answer will be available soon.