Nothing is 100% safe. So it comes down to how much risk you are willing to take. Using public wifi has its own pitfalls and things to watch out for. Following the advice and precautions shared by other FTers will definitely help mitigate that risk.

Personally I try to avoid doing bank or important business transactions in public places. This is probably more of a personal comfort though than the reality of what could happen.

The Reality is this: You are much more likely to have your CC information stolen at Wal-Mart than over a Wifi connection. Nonetheless I wouldn't throw caution to the wind.