Sorry, but you said you got it at Best Buy? If you are under the 15 days, 30 minutes of talk time and what ever the data is, they can't say 'no' to take it back. They have no way of knowing it is unlocked unless you told them (and why would you?).
I bought a Bell Galaxy S2 and did a SIM unlock (paid bloody $40 for it as 'rooting' it and doing a free unlock would have voided the warranty and made it noticeable as you get a big yellow triangle on boot up apparently), found it didn't work on Rogers network (stuck in Edge and GPS didn't work) so returned it. While the guy made some comment about unlocking voiding the warranty (it doesn't) I didn't bother getting into a discussion with him or tell him about it.
Best Buy is a lot less picky on returns than Rogers... who take the time to actually check our talk time (who cares what your talk time is... especially if you are already a client... just some B.S. policy they made up to reduce the number of returns). Best Buy has never actually checked the talk time, though they ask the question. Mind you that is me buying the phone outright... maybe if it was a new contract I would assume they would check.
Anyways... if you are over your 30 minutes of talk time and that is why you can't do a regular return, you can always do a warranty replacement and you'd likely get a new one since it just came out (I got a new 9780 from Rogers when I complained about reception issues with the phone).