I've done it and it's saved me.
I had an iPod Touch and no working cell phone while down in NYC to meet someone. Unfortunately it took me longer to reach the meeting area than I thought it would and I missed her. Yes this was a date.
Anyway, using open Wi-Fi signals I was able to send her an e-mail and let her know that I didn't chicken out but was late.
If I had more control over my wi-fi signal I may even set it up so that 1% of my bandwidth is open to anyone who needs it.