Something like node.js, narwhal with wsgi or ringo.js? On a really basic level, coding the front and backend in the same language has really saved my little brain quite a bit of frustration, especially when I'm using plus's to concatinate in php and dots in javascript. I work with node and know that a properly architected event driven server can be wicked fast (not just fast, but WICKED FAST!). There is quite a bit of middle ware available and why not work with mongodb in the backend to create a perfect trifecta of javascript?
node is really simple to setup and test with apache bench (ab), narwhal and ringo took a bit longer for me (I was trying to run them in google apps engine though).
Curious what you end up going with.