It seems that you can't browse with just wifi anymore by default. I bought the flip to use with wi-fi many months ago, but would not spend $25 a month for a data service. It took some work, but I got wifi to work on the Flip.
It was a simple as configuring the browser to use the hotspot browser.
Then came the 503 error to which the supposed solution is to reload the software.
This killed my wifi browsing.
I wasted some time and gave up.
Today I decided to waste a couple hours with T-Mobile support.
The third call yielded an intelligent support rep
who explained to me that they allowed the wifi access for a while and now require a data plan. Because my phone once worked and no longer worked, she explained why this happened and how to work around it.
She said that the wifi browsing looks for a "service book" before allowing browsing. I lost that "service book" by updating my software.
To add the service book, you can add the data services and then cancel them.
This should allow you to browse the Internet with wifi. It hasn't activated yet, so I can't verify this.
Of course there must be some way to put a "service book" on a blackberry, at least an unlocked one, with the right know how. For me ordering and cancelling is a good enough solution.