My experience is not directly relevant to 3G, but similar....
I spent a few weeks in Toronto, Canada with an Edge-based iPhone and AT&T's International roaming plan. When I got my bill, I discovered that:
1. AT&T's data plans are pro-rated (my bad for misunderstanding this)
2. AT&T's charges defy any form of reasonable audit
The second point is the rub. Despite the fact that I was only and always in Toronto while in Canada, AT&T's data roaming charges showed locations from Vancouver to Nova Scotia -- often within minutes of other data roaming charges. As I pointed out to AT&T, it just isn't possible to travel 3,000 miles in 20 minutes. The Canadian roaming charges also included times when I was at an airport in the USA -- and despite my offers of evidence to AT&T that I could prove where I was, AT&T's CSR claimed that all the charges were accurate. I eventually had to escalate the dispute to my local Better Business Bureau and it took many months to get straight.
My opinion: AT&T's back-end billing systems are a big part of the problem, coupled with the fact that AT&T's front-line service reps have no authority to over-ride charges that clearly defy reasonable logic and evidence to the contrary.