Can you unlock any iphone 4 to any carrier?

I saw on eBay that some say locked to AT&T only can't be unlocked. Does it mean you can't at all even though you jailbreak it?

Jailbreaking doesn't unlock iphones.
only baseband 1.59 can be unlocked on the iphone 4 and none on the 4s.

Jailbreak and then unlock it.

It you want an iPhone 4 that's unlocked, you should buy one already unlocked. Most used ones can't be unlocked right now