Since HP recently completed its acquisition of Palm, the move makes a lot of sense. Here are the OSes that may play into HP's slate plans.
WebOS: HP executives have repeatedly said the Palm acquisition was about much more than smartphones. The Palm deal was really about tablets and a wide range of devices. HP sees a mobile stack across multiple devices. Simply put, HP is doubling down on the WebOS and most pre-Palm plans for tablet PCs and slates are up in the air.
Microsoft Windows 7: Windows 7 slates will hit the market and Microsoft has other lightweight operating systems on tap. Microsoft's grand mobile plan is muddled to say the least, but HP will give the software giant some time. Why? HP and Microsoft have too many valuable partnerships elsewhere. HP will play ball with Microsoft-powered slates.
Android: You don't spend $1.2 billion on Palm and then go with Android to power your devices. HP may ultimately have an Android flavored device, but where's the differentiation? Everyone and their mother will have an Android tablet. HP will keep a toe in the Android market, but clearly sees the WebOS as its mobile meal ticket.
Given those moving parts, HP's move to table Android only makes business sense so it can focus.