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