there are a few websites out there designed to test the iPhone's internet. Hell, you can use cnet's internet speed test too. The app store also has a few .99 programs that you can download to test the speeds.
In the Chicago area, I haven't gotten anything over 600kbps on 3G. But thats a hell of a lot faster than the 40kbps that I was running on average with EDGE....
Is the Tilt on the same network in the same area at the same time?
I'm seriously planning on asking AT&T for a credit based on lack of reliable 3G. I'm paying for for high speed internet, the same or more for anyone else using other phones on the same network. Often it's no faster than EDGE. Refund.
my suggestion is to not go by network speed test, go by real world performance. I regular get edge speed speed tests but actual page loading is generally 2x as fast. I also find that the real speed inhibitor is the phones processor. Although I have 30mbit/s Internet, the phone only loads a few seconds faster than 3g.