Last week, on the PHP-DEV/php-internals mailing list, Jakob Santos proposed a JIT compiler for PHP. Rasmus responded with (paraphrasing) 1) a JIT won't help PHP and 2) PHP doesn't need a JIT, it's just glue code around functions written in C anyway.
Some people countered these arguments. :-)
And then yesterday, Jakob posted the beginning of the journey to his blog.
Interesting; I look forward to following this work.