I think the general price for roaming data is $19.50 per MB in most
countries, though I think you can get a 100 MB allowance to use in some
countries, plus unlimited Internet in the US, for $140 per month or
something like that.  I have occasionally considered taking the latter
plan when I had a trip planned, and then canceling it before the 30 day
trial is up (it's a two year contract otherwise), but I've always decided
it wasn't worth it.

Roaming data with a mobile phone is expensive.  A better idea, if you
are going to be in a country for more than a few days, might be to look at
the Internet options available with local prepaid service which you can
buy when you get there.  Unfortunately, while the prepaid services in
some countries have very nice, reasonably priced data services for prepaid
SIMs, many others have none at all (the US is among the latter I guess).

In general, unless you have a huge pile of money and/or an intense need
to get Internet service on your phone no matter what, I find it is
usually better to think in terms of hotel Internet service, or Internet
cafes, or WiFi service if you can find it.  In a few countries you can
find nice data service with a prepaid SIM, but roaming data with an AT&T
SIM is too rich for me.

Dennis Ferguson