Does anyone know of any decent sources of information on network latency in the mobile environment?

I searched the web and found some suggesting that (at least the 2.5G networks) were not materially worse than dialup (250ms-500ms). I spoke to some people more familiar than I with developing mobile apps, and they told me to expect numbers more in the 1-2 seconds range. Our early testing suggests that the number might be closer to twice that (round-trip ping times between phone and server).

Aside from decent data on mobile network latency in general, I'm also looking for information on the key factors affecting the results. For example, does the network latency vary substantially by carrier? Lastly, are there technological advances over the horizon that will dramatically reduce mobile network latency?