I am a mobile apps developer who has run into some strange and interesting problems with 3G connections. We develop applications using both the Twitter API and Twitter Streaming API. The normal Twitter API allows 150 requests per hour from one IP. This limit is cleared for every IP at the beginning of every hour (e.g. 3:01 PM).
What we experience using AT&T's 3G service on the IPad is, no matter how many new IPs we get toward the end of the hour, usually somewhere around and after (e.g. 3:30 PM), we get rate limited. My theory is that AT&T actually uses the same IP's for multiple devices and is routing the requests through a central computer. So the device doesn't really have it's own IP Address. I have no idea how these systems works or weather I am even on the right stackexchange, but is this theory correct?
There is no way our device is hitting the 150 requests an hour. That would literally be almost 2.5 requests a minute. The device always gets rate limited towards the end of the hour even with an IP change.