This sounds exciting.
I think I'll be taking a closer look but in the meanwhile starting a thread to discuss this thing.
Article synopsis - HipHop transforms PHP code directly into compiled machine code - it does this by first turning it into C++ code, then compiling that. You don't have to alter how you write PHP programs with one exception - you can't use eval (Understandable since eval requires an active interpreter to function). They claim a 50% boost in speed over normal PHP with APCaching (which itself is 5x faster than standard PHP, so a 2500% overall boost from normal PHP without caching).