erm HP already has been competing with MS for a while, they push HPUX for workstation class desktops (for business class OS it's actually pretty sweet, especially when it comes to computational intensive applications), webOS would allow them to crack the sub-workstation class desktop, all evidence points to a HP assault on the corporate sector, once they mature webOS on the business sector they will likely refine it for the consumer market although probably still offering it as a consumer option while refining it
look at RIM, they survived for as long as they have only because of the corporate market, if the product is good you can happily sustain it on enterprise sales alone, and as much hype and stories we hear about the iPhone they yet to really crack the corporate market (lack of any real security and user control over the device is really hampering them there), if HP plays their cards right they may well be onto a winner without really needing to take away market shares from the iPhone (which is a hard feat)