The power output of the 3G transmitter is quite low, although I've never seen the specs.
I haven't used the iPad in this manner yet, but the iPhone gets almost too warm to hold comfortably when I run a GPS app and charge it at the same time (which is almost a necessity with the iPhone - the GPS seems to take a LOT more power than the 3G radio.) it seems to get to "full heat" in a half hour or so. I've had it set up that way for several hours at a time. It seems to get hottest on the upper back.