Not to piss on the original posters party, just because you found it does not make it theft if you fail to carry out "due diligence". This means that you must exhaust all avenues to attempt to find the owner the property. This means contacting the carrier, you have a way of doing this, the phone will still go through to the carrier and you have already mentioned you have used *611 already and gotten through.
I think you know that it has a bad ESN because I think that, no I am 100% sure, because of what you said, that Customer Service have told you that it has been reported missing to them and they have blocked the phone temporarily. I am also pretty sure that they asked you to return it to the nearest Verizon store, just from what you have already said!
Myself and I know others are already blatantly aware from what you said that you feel that because you "picked it up" that it is you phone now. It doesn't work that way.
Any bets that your next avenue would be to try and sell it on ebay or craigslist on the basis that you bought it and didn't like it and try and palm it off onto some sucker.
You really did come to the wrong place for advice on activated a lost or stolen iphone. Just the same as it would to go to a crackberry forum for a lost or stolen Blackberry.
People pay money for their phones and you should at least make a whole hearted attempt at returning it to them and you don't live a million miles away from a verizon store. If you are uncomfortable about too many questions at the verizon store then put it an envelope with a note saying you found it and couldn't locate the owner and mail it to the local store. At least then you will have a clear conscience that you have tried!
What you have already said makes it blatant that you haven't even tried and believe that opportunity has knocked!
I'm sorry for you. Not as sorry as I am for the person that lost (or ???) the iphone!