In which countries can I use 3G service with a Wi-Fi + 3G model of the iPad 2 that was purchased in the U.S.?
A GSM iPad should work in any country where GSM carriers operate and offer data plans.

mobileworldlive.com is a great resource. They have a list which is up-to-date with live and planned availability as well as supported frequencies.

Of course you can use your iPad in any country where your carrier has roaming agreements. But at a very high rate. And you might have to call your carrier in advance to activate the option. But this will cost you much more than getting your data access from a local carrier.

If you chose to get your data access from a local carrier here are a few things to consider:


The carrier will not necessarily offer a micro SIM. In that case you will have to trim the sim to the right dimensions yourself.
You will have to setup the APN settings yourself on most networks.
Internet access could mean something else than data plan. Beware of obsolete offers with WAP or very limited access.
Some carriers will have better coverage than others. Check the coverage of each carrier before committing.
Some carriers will not sell prepaid data.
In some countries there could be restrictions for foreigners or non residents and you might have to get your SIM from someone who can buy it for you.



  Does this mean that any iPad 2 purchased in the U.S. is incompatible with 3G service abroad?


No. The GSM version (AT&amp;T) will work in most countries. The same cannot be said of the CDMA version (Verizon).

Wikipedia tels us the iPad supports UMTS / HSDPA (850, 1900, 2100 MHz) GSM / EDGE (850, 900, 1800, 1900 MHz)

Two networks are referenced as providing support for GSM based 3G in japan :


NTT DOCOMO, INC. (DOCOMO) 3G on 2100 MHz
SOFTBANK MOBILE Corp. (SoftBank) 3G 2100 MHz




Note: I would recommend a 3rd party solution if you go abroad for a short period of time. There are mobile WiFi modems like the huawei-mifi-e585 that could offer a better alternative if you need internet access on more than one device at a time.