THis is an old thread, but it bears enlightening some newer folks about international data roaming.
As has been said before, with the current iPhone software all you need to do is turn off Data Roaming. You don't need to turn off the phone, or turn of EDGE or put it in airplane mode. Simply turning off Data Roaming in the Network Settings of the phone will keep you from running up a big tab. If you disable Data Roaming, no data transfers take place. Period. You can still use your phone as a normal phone.
Now apparently there's an issue if people call you and the call goes to voice mail. I've read that can result in a charge as if you had picked up the call. I haven't had the issue happen to me yet, but this is separate from the original data roaming charges anyway. The only way around this seems to be to turn off the phone (power off, Airplane Mode, etc). The other thing that is expensive is text messaging. AT&T charges $0.50 per message (both incoming and outgoing) while roaming abroad.
By the way, I find that just a few pages of web browsing can result in many MB of data transfer. It is possible that the OP just went to one or two pages and transferred all the 5.3 MB of data. At close to $0.02/kb it is easy to accumulate a $50-$100 bill even with just a page or two of browsing. We just don't think how much data web browsing uses. The rates for international data usage are atrocious at close to $20/MB.
When I go to Europe I turn on an international data plan through my AT&T online account manager. It costs something like $25 for 20 MB of data transfer and there are plans for more data usage. I keep data roaming off most of the time, but turn it on to download e-mail from time to time and to use instant messaging as needed. I have my e-mail set to only get a portion of the messages so if I get a large file I don't download it through the cellular network. In Europe many hotels charge up to 25 Euro/~$40 for 24 hours of internet usage. Having the data plan helps manage my costs and relieves the need for WiFi access every day. There are also free hotspots available, but it takes more effort to find them and I am not always sure of the security implications of using some of the places. I do use them when at a reputable location. I like the additional flexibility provided by the international data plan. Occasionally I do need a quick Google search and other light web services and it is worth the $25/trip to me to have 3G access.
Now all I need is for Skype to update its application so I can use VoIP over the 3G network and I'll be all set.