Sounds to me like they should have just called it what it is…Edge 305 Cx…I would highly doubt that they are going to use a completely new software platform for these new Edge units. It is more likely that they’ll function more like the higher end eTrex units with the added functions that the Edge is known for.
As far as basemaps, you can build and load your own with software such as Global Mapper in conjunction with cGPSMapper/MapEdit using the homebrew techniques listed at various blogs and forums. OR you can buy the software on DVD/CD or preloaded onto the microSD cards.