There is a lot you can do to make using public wifi very secure.
The risks of using public wifi are:
Data/password sniffing - everything you do that isn't encrypted (i.e doens't use SSL) can be sniffed by anyone. If you use an SSL site though then you are safe as long as you take heed of any certificate warnings you get. This means that things like Gmail, Facebook and online banking are perfectly safe on public wifi (if ssl is enabled in the site's options).
So you can either use SSL only web sites, or secure everything using a VPN or similar secure proxy. Assume that any other unencrypted traffic is being watched and you will be fine.
The second risk is that someone could exploit your phone while you are connected to the wifi network. If this happens it's game over, but what are the realistic chances of you ending up one the same network as someone with these skills? It is a risk though, there are recent vulnerabilities in Android for example that are trivial to exploit and an attacker could force you to visit an infected website.
So basically use SSL sites or a VPN tunnel and you're fine 99.943% of the time