AT&T just changed their int'l data plans last month to a much more reasonable level for 3G service:

120 MB Data Global Add-on package for $30/month
300 MB Data Global Add-on package for $60/month
800 MB Data Global Add-on package for $120/month
The overage rate for all packages is $30 for 120MB.

I've been paying $25 a month for 50MB for my iPhone, and haven't gone over. I don't use voice internationally - except Skype of Google Voice. But I use email regularly. Also there is an international text plan - inbound messages at your normal rate (I've got unlimited domestic) and 50 outbound for $10.