Great Question Alice - Not sure which situation you are in.. Is this a company that hires in a central headquarters for both locations - or would your interview process be completely different? (I mean, are the people you interviewed with able to hire you for either location or would you have to meet and get an offer from totally different set of people).
If its the same group - just be honest and talk to them about your preference but state it as a preference, NOT a requirement. If its not the same then it becomes a bit more risky. If its not a big diff in commute and you are excited about the job, I might suggest just getting that foot in the door. If it is a big commute, then same rule applies as above. Honesty. Have a discussion about how excited you are to accept the position, but it would be a PREFERENCE NOT A DEMAND to work in the closer location - would that be an option. If there is resistance to the idea - take the offer in front of you and then reinforce how excited you are about it. This should give you best chance of possibly switching but not losing the job.
And on both counts - better to do this in person than on phone. I wouldn't make a big deal of it. Just suggest you will be in the area and can you stop in with a couple quick question....

Thats my two cents - any other Jobcasers have thoughts? Hope this helps and CONGRATS on the offer in hand - thats great news! Fred