Shumway, although it suffers from a terrible name, has the benefit of Mozilla's funding. Gnash and Lightspark don't have have that luxury, but they do have the benefit of years of development. I think that, like Nathan, that Mozilla's project will probably not see the same level of adoption.
Mozilla has the option of promoting Shumway by bundling it with Firefox, but it would be unwise to turn it on by default since it's unlikely to work perfectly. In order to prevent a backlash, they would have to have it be opt-in - and even then vast majority of users are going to prefer Adobe's implementation because it will work, and Shumway may not.
Really, the people that Shumway may serve are Linux Firefox users that don't want to run proprietary software. The problem for Shumway is that Gnash and Lightspark already serve them pretty well. Also, a small market like that is unlikely to keep Mozilla putting money into Shumway if it won't get widespread adoption.
Shumway sounds interesting, but it's likely to not garner much of a following. Mac and Windows users will continue to use the proprietary plugin, and Linux users already have "good enough" existing support.