There are still a few possibler roadblocks:

Device drivers
Even if they release source of the whole OS, it is unlikely they will release source for all of the device drivers as its often provided by third party hardware vendors. Even if they did the very unlikely and did release the driver source code, it would cover just the current WebOS devices - porting to other devices would still be hindered by missing drivers.

GUI Toolkit compatibility
WebOS uses Mojo and Enyo for most of the apps - both is html & javascript based and will be likely opensourced. there is also SDL (even with PyGame support !). Other than that there is currently no support for other toolkits like Qt, GTK, Clutter, EFL, which might be putting off some developers that don't want to learn yet another technology usable only on a single platform.

Still, this this one can be probably solved by the community once the source is released, but might be hampered a bit by the fact that WebOS works directly with the framebuffer, without running an Xserver.