Sniffing Public Wifi

Hi All,

I've Googled the question with mixed results. I just wanted to know the opinion of a community that knows much more than the average bear on a topic that seems to be a bit controversial.

Is it legal to sniff traffic that's public?

I realize that if you sniff some valuable information on accident, a responsible person would not disclose the information. Currently, my apartment complex has a free wifi so my rent covers it so I would think I could sniff that traffic. However, what about Starbucks? Is the act of sniffing legal?