Also, they created Hiphop so they could keep using PHP. =p
If they were just going to write everything in C++, why wouldn't they do that from the start? Hiphop isn't meant to convert PHP source to C++ source so they can write C++. It does it so they can still write PHP, but get the benefits of a truly compiled language.
The big problem with truly compiled languages, which a lot of people forget when they get into this argument, is often times you have to replace the whole site in order to make the smallest tweaks (because generally have one or several large executables or libraries which your code was compiled into). This can be difficult if you aren't already working with a cluster where you can roll it out onto one and let it propagate without causing downtime. With PHP, if I need to make a tweak in just one file, I can and nobody would notice except for that fraction of a second moment when the file is actually being overwritten.
Compiled websites aren't always the answer either.