I would think that we will see at least one more model,
first, these doenst seem to add much compared to the existing models, sure the 300 has a smaller screen but that isnt really anything that define progress
second, the modelnames, it is pretty uncommon that a company release a new version with a modelname that sounds like it is a lesser model than the one it replaces, the 600 is prob replacing the 505 and I think and hope that we will see a more "high-end" model that will replace the 700 (800 or 900 perhaps
)
My guess that these two is the budget/mid models, the 300 is cheap because it is a small screen and the 600 is prob a reworked 700 and will prob have the same price as the 505.
just my 2 cents
*edit the link that Sweetpea provided to the prs-900 looks really fake
