Dumb question, but what's in it for HP?
Now, I'm not saying this will happen, but at least it's possible now. webOS wasn't going to do anything for anyone sitting in a corner. Personally, I think a lot of its success depends on if/how they manage to set up a competitive webOS app store, facilitate at least one device that can run it, and letting webOS run as openly as possible on that device. Open source advocates would flock to buy and develop for a device running on a truly open OS with an open development model.
I'm also curious about a compatibility layer for Android. Dalvik has already been ported to maemo/meego, although it's closed-source. I'm relatively sure someone will now port Dalvik to webOS. I think a webOS with a vibrant, open-source community, able to run Android apps, could pose a significant challenge to portions of the Android market (to iOS to a lesser extent).