It also seems to depend on how many "bars" you have. When using 3G, if I have 5 bars, i consistently get >= 1.0 Mbps. On two bars (in the same building but away from windows) I get 0.5 Mbps.
First off I have 3 tests performed on iphone 3G they are as follows
1) Cable modem (COX) to airport extreme WiFi = 1598 kbps/ 135 kbps
2) EVDO modem tethered to my MBA then connection sharing over airport to my iphone = 1100 kbps/ 73 kbps
3) iphone over 3G network in southeast Louisiana 623 kbps/ 95 kbps.
I also tried my EVDO modem through a PHS300 (cradlepoint) results sucked 400 kbps/ 80 kbps.
I don't know what happened, either a glitch or a surge lol.
My true average speed have gone up since last week though, seems like the network is starting to stablize. I get around 100KB/s now on 2-3 bars. Though, i still do occassionally get around 30-40KB/s.