From what I understand, the only difference between a locked and unlocked phone is that the unlocked phone, it can work with any carrier's SIM cards. When you purchase a locked phone, say from AT&T, the phone is "locked" onto AT&T's network. But you can obtain unlock code (either through internet or sometimes your carrier) to turn your locked phone into an unlocked one. The unlock code is just some combination of keys you need to enter. I hope this helps.
I suppose you can use it on any GSM supported networks with their SIM cards. That includes Cingular. I am actually looking at couple other GSM phones as well including Motorola v600, SonyEricsson p900 and Nokia 6600.
i have several gsm phones. a couple of which i bought of ebay unlocked. In a nutshell: Every GSM provider locks their phones to their own service. If a subsidy unlock code is obtained the phone can be used on another GSM provider's network (assuming you get service and a SIM card through them). THe only other issue to be careful about is which bands (frequencies) the phone implements. Some european phones only are dual band with the 2 frequencies for eruope/asia (900 and 1800 mhz). The V400 however, is one of the 1st quad band (all frequency) phones which will work anywhere in the world. a.k.a. "world phone".