It's really easy. Just a matter of unlocking your device, removing the csc, and dropping in an apns-conf.xml from the international version. If you aren't comfortable Modding your device the international version might be better if you plan on traveling a lot.
Please do post a step-by-step. From everything I've experienced while travelling, it seems the only way I've been able to bypass and manually select in the past was when I had infuse and I'd used your custom ROM (Infused) in conjunction with the Rogers Radio (rogers doesn't limit their customers' ability to manual selection menu). Failure to have been using BOTH your Infused 2.3.3 AND the Rogers radio would lock me out of the Manual Network Scan menu, and just grey it out to "Default" which=whatever ATT wants you to be attached to.
If you do know of an alternate way to re-enable that menu, please do share.
I ended up taking the leap and getting one of the i9100's (in nice looking white) while I was in Thailand this week. I was so excited to have a phone without carrier modification!
After setting it up and using it for a day, I just can't get over the damn physical key in the middle of the phone. I feel so weird using the button, and after having the HTC Inspire, then Samsung Infuse, and then SGSII ATT version, I just can't go back to a sloppy physical key, so I eBay'd and sold the new white one and went back to my old ATT Galaxy S II in favor of the four keys. Now I just need to work through the issue of not being able to manually select a carrier's network.