Global use depends on the phone and your carrier. Most phones on ATT should work in Europe, as should any other GSM-based phone, but the usage rate without an international plan is really expensive. Blackberrys should work overseas but it’s always best to ask and be sure before spending money on one.
Only Verizon’s 4G phones will work overseas and definately ask before buying.
Btw, Verizon’s 4G network sounds to be pretty epic. SIM cards, longer reaching towers, and during initial testing they were seeing 50-100MBit/sec downstream speeds. GG