The Pre can use WiFi as an alternative to EV (or 1X or...) data for receiving/sending e-mail, web browsing, calendar/contacts sync, etc. As the previous poster said, it cannot be uses for SMS (or MMS) or to make phone calls.
Originally Posted by sciteach
Basically, it's an alternate way to "do data." In theory, it's faster. And works in places where you can't get a good data connection.
You'll need to have a WiFi router, of course. And be in range of it.
The nice thing on the Pre is that it'll switch between connections automatically. So, for example, I can walk home using EV. Then, when I come inside my house, it picks up that I have a WiFi router and switches to that. Later, I can go back on a walk and switch back to EV. If I go to a coffee shop that has free WiFi, it can switch to that when I go inside.