With 3 overseas trips totalling 10 weeks over the next 4 months, we were all set to switch to T-Mobile and take advantage of their unlimited text and data plan. However, the sales person warned us that another customer went to Europe soon after switching and had his service cut off because T-mobile assumed from the international use that it had been stolen. He said there's no hard and fast rule, but they use an 70/30 rule of thumb.... you need to be using the phone in the US for 70 days before a 30 day overseas trip. He didn't know if it would work to alert them about the foreign travel the way you do a credit card.
This seems nutty since many folks choose T-mobile for the international plan, but we weren't willing to risk it so we're sticking with Verizon for now and I'll use an international SIM in my phone for local stuff.