The question is, did you return the right one? Did you check the IMEI on the receipt against the one you kept? Both have a unique ID, (listed on the outside of the box & under the battery) and I'd have made sure to keep the one they thought they sent me.

Hard to say what I would have done... Depends on how they company treated me in the past. Since my provider has been pretty good about things, had I gotten it through them I would have returned it. If not though, I may have sold it to someone to re-sell on the streets of a foreign country? (There was a thread about that not long ago, btw...)