Yes WebOS has not done well with the public. The reasons have been discussed to death here(marketing, Sprint, hardware,...etc). I think HP sees WebOS on a tablet as being a perfect way to reintroduce the OS to the public. I think we all can agree that WebOS will work great in a tablet form. In a way that may be the ideal platform for WebOS.
The tablet world has yet to be overrun with products and OS's like the phone world so if HP can get a WebOS tablet out sometime later this year, they could position WebOS as a premiere tablet OS. If they are able to get a hit out of that product, that would be the key to helping WebOS succeed as a phone OS too. Palm via HP might have to go the reverse route of going from bigger to smaller platform to become a success in the phone world. Well that's assuming that the name Palm will still be used.