There are actually people who will pay you real $ for your old one, assuming it still works and is in decent condition.
CellforCash.com buys more than 10,000 used phones a month, then resells them in Latin America.
OldCellPhone.com buys more than double that.
TradeMyPhone.com will either pay you cash or take the phone as a trade-in for a newer model.
They pay between $1 and $150 depending on the model.
Here's how it works:
1) You fill out an online form with the model number
2) They send you a prepaid box in which to return the phone and battery
3) You get a check in the mail
I'd bet that if you were really serious about this, you could make a fair amount of money getting people's old phones and selling them to these companies.
If I was a kid, this would definitely be my summer activity. Fun + profit.
That's a hard-to-beat combination.