According to DigiTimes and its unnamed sources (so take this with a bit of caution),
The development of WebOS tablet PCs was finished in 2010, but HP will have Taiwan-based Inventec begin ODM production and shipments in March 2011 to allow for software testing and timing of launch schedules in line with market development.
In the leaked documents and images yesterday, signs pointed towards a mass market launch plan starting in July. Presumably this sooner launch schedule could be for developers, etc. Or maybe not. We'll know more on February 9.