The PlayBook ships with a recent version of WebKit and all Blackberry 7.X phones contain a WebKit based browser. Starting with the Torch last summer most BB phones shipped with WebKit. The days of the Java based browser on BlackBerry's are over.
Please correct me if I am wrong, but that is not the main benefit of WebOS. I believe the main benefit of WebOS is that it supports HTML5/JS applications natively in the OS, and they are not bound by a browser frame.
Nobody wants to have to open a browser and load a bookmark to get to an application that has browser widgets taking up space at the top, bottom, and side of the screen. They want a home screen icon they can tap once and get an application with native look and feel. If developers can write that app in HTML5/JS then it's a win/win for both developers and users. Developers get a highly portable framework that allows them to rapidly develop software and users get a native app experience.
Disclaimer: I am part of the WebKit team at RIM.