If HP wants WebOS to be any sort of success, they'll have to license it to other hardware manufacturers. WebOS doesn't have the 'deep time' in the market like Apple's systems have.
Palm tried for a while to license their OS to other companies, then made the colossally idiotic move of selling their OS to another company so they'd have to pay to use their own creation. To cap off the stupidity, Palm then wouldn't adopt Palm OS 6 and since Palm wouldn't none of the other Palm OS device makers would either.
Apple is a textbook example of how to go about being a sole source of both hardware and OS.
Palm initially was, then stayed a success while also licensing their OS to others, even re-branding some of their hardware. Then Palm got stupid, the creators left and formed Handspring, Palm split into hardware and software divisions, and somewhere in there the creators came back, Palm OS got sold off to Access, everyone quit making new Palm OS devices, Palm started making Windows Mobile devices (imagine if Apple announced a wPod running Windows Phone 7!) and just generally punched big holes in the hull of their sinking boat until producing WebOS and the Pre - too late - unless HP can shuck the stupid and do the hard stuff that will be required to save it.