It appears as though AT&T has put a cap on the 3G upload data speed in certain regions. Members of MacRumors forums are reporting that the cap seems to be at 100 kbps and the areas that are affected are:

NYC, Central Jersey, Boston, Orlando, Seattle, South Jersey/Philly, Columbus, Cleveland, West Houston, Phoenix, Northern Colorado, St. Paul/Minesota, Suffolk County/Long Island, Quad Cities, South Jersey, Denver, Detroit Metro, Cincinnati, Baltimore, Salt Lake City, Las Vegas, Kansas City, Fairfax, Minneapolis

A possible explanation could be maintenance work related to HSUPA (high speed uplink), the network protocol that will bring us faster upload speeds to devices that support it, like iPhone 4. We hope to hear an official statement from AT&T soon.

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I did a quick test in Denver while connected to my AT&T Microcell and got a whopping 47 kbps upload speed. Meanwhile, Jeremy is experiencing no problems in HSPA-drenched Chicago. What about you? What 3G upload speeds are you getting and where are you located?

[Gizmodo, Macrumors forums]