I just looked at the logs for my main mobile data/Internet smartphone. In the last 30 days I've used half a GB - 480MB download, 40MB upload. Probably 70% of that has been over 3G, with the rest on my home WiFi.
And that's without doing anything too fancy - no YouTube, no streaming audio, no VoIPo3G, no P2P. Just normal web browsing (a mix of mobile and "real web" sites), email, RSS, plus a fair amount of Google Maps.
Yes, I'm quite a heavy user - I'll update my RSS feeds while I walk to the tube station & read them on the train, and I'll click through to content-rich web pages that are 500k-1MB each to fully load. And I've downloaded a few PDFs of reports and other documents as well. But I probably have the device in my hand as much as a typical BlackBerry or iPhone owner that I see around London.
And this is with fairly lacklustre 3G/HSDPA performance. I don't think it gets above 1Mbit/s at all, and I'd say its considerably slower most of the time. And on a device which has a UI I'd give 6 or 7 marks out of 10.
My cellphone data usage has gone up roughly 50-fold with the advent of flatrate data, HSDPA and decent browsers.
With an iPhone 3G, or an S-E Xperia or a similar device, with a real 3Mbit/s+ peak speed, I could certainly imagine doing 2GB a month, maybe more if I decided to watch video podcasts on the tube rather than text RSS, or streamed Internet radio. And if I used the phone as a tether for my laptop rather than having a separate dongle, you could probably add another 1-2GB.
OK, I'm probably a top-1% user at the moment. But I'm definitely wondering whether we're going to start seeing some real congestion issues on urban 3G networks sooner than we expect if sales/use of upper-tier devices really kicks in.