The Z600 is a GSM phone, so you'll need a GSM carrier. Currently T-Mobile, AT&T, and Cingular are the big U.S. carriers that provide GSM. AT&T and Cingular also have an older TDMA network that they're trying to transition people off of.
The so-called "smartcard" is more commonly referred to as a "sim card". It contains your basic cellular account information, and sometimes your address book as well. So by moving it between phones, you can switch phones with great ease.
Phones purchased through carriers will be locked to the carrier's own sim card. Each carrier has a different unlocking policy. T-Mobile's is pretty liberal. AT&T refuses across the board I hear.
Their are third parties that will unlock a phone and various tools available for free or for purchase on the Internet.
I took my SE T610 to Europe in December, and I could purchase pay-as-you-go sim cards there, giving me a local phone number. That came in very handy.
And yes, Bluetooth is wonderful!