It will use the wifi on the playbook first, if not connected there, it will use the data on the phone and if the phone happens to have a wifi connection it will route it through the wifi on the phone, just like the phone does.
A small benefit to this phone connectivity is that you can put your phone somewhere there is better cellular or wifi coverage and then bridge to use the bridge apps or web browser on the Playbook. For example, I was away this summer and the cellular connection only worked when higher up. I put the phone in a higher spot and was able to use the internet on the PlayBook.
