Nice idea - so start designing the board now!
Personally I'd not hold my breath the Prop II is a complex beast and its taking its
time to get finalised (tiny development team for such a chip, but with an army of
alpha-testers let loose on the FPGA models)
There is an existing Prop I board (Propeller-ASC) compatible with the Arduino
shields, and I have a vague recollection someone's written a software compatibility
layer too (?).