Swapping S cards on--go iPhone

Assuming my iPhone is unlocked, I have a question about swapping SIM cards if I travel. With a traditional cellphone, I can simply swap SIMs when I land in another country and the phone works. I get the impression in these forums that with an iPhone I would have to connect to iTunes and reactivate after swapping the SIM card.
Is my understanding correct? Do I need iTunes to activate the phone after swapping?
Suppose I travel with a computer other than my usual syncing computer. Can I enable the phone without wiping out my apps and music?
Thanks
Home Brew Windows Vista iPhone + iPod Touch 16GB + iPod Nano 4GB