IIRC, TKIP generates a seperate key for each client based on the master key, the client's MAC and some other random salt. So, it can be sniffed, but must be decrypted. That is if I remember TKIP correctly

/edit oops, just saw this which kind of negates what I was saying
WPA-PSK (TKIP, with RC4): B also is able to decrypt A's traffic (same PSK, MAC known, ANonce/SNonce known -> PTK predictable[2])