According to a report on Digitimes.com, the upcoming Pioneer 400GB Blu-ray discs will be compatible will all current Blu-ray players, including the PlayStation 3.
"The technology of the super multi-layer read-only disc is based on Blu-ray Disc with a breakthrough in material of reflective layers. The specifications of the pick-up head (PUH) of the disc is the same as those for the PUH of blank BD discs, and therefore the Pioneer discs can be read on BD players." - Pioneer High Fidelity Taiwan.
Apparently, Pioneer plans to have read-only 400GB Blu-ray discs on the market by 2010, with rewritable discs in 2010-2012 and 1TB (1,000GB) disc in 2013.
Stay tuned for more information as it breaks.