Does Starbucks use WPA2? The last one I was at was open, I just had to click through a terms of service page.
If you are sniffing the 802.11 frames (and you should assume someone is) and you catch the entire 4-way handshake and the nonce generation is predictable you could reverse-engineer it, but then again you can say the same thing about a TLS connection too.
The whole standard it a mess; it should have opportunistic encryption on open networks, then clients can display a warning if this doesn't happen for whatever reason ("Anything you send or receive over this network may be readable by others" or similar).