VoiceStream Wireless - now T-Mobile - may not have the best coverage of rural areas in the U.S., but as the leading provider of GSM (the European and Asian standard for wireless systems) cellular services, it's the clear choice (no pun intended) for anyone who wants great mobile phone and data service, both in the US and abroad.
Making sure you get service where you need it requires a little homework - not every handset they sell is capable of operating overseas (due to differences in the frequencies used by different countries). But if you buy a tri-band phone (or, if you want to get fancy, try the new 4-band consumer-oriented Blackberry phone / email device), and make sure your service contract is enabled for international calling, you can use your T-Mobile phone pretty much anywhere in the world where cellphone coverage is available.
One other nice thing about T-Mobile is their pricing. When compared to the other leading providers (AT&T, Verizon, etc.), T-Mobile is consistently 10-15% less expensive - not a huge deal month-to-month, but if you expect to have a cell contract for the forseeable future, the dollars can add up.
But don't take my word for it, drop into a local T-Mobile store (or go online) and see for yourself!