In a new post to his blog Hannes Magnusson mentions that the PHP packages for Ubuntu linux installs are woefully out of date and can make a developer's live even more difficult than it already can be. It help ease the situation a bit, he's figured out how to provide custom PHP 5.3 packages to anyone who wants them through a service called Launchpad.
Launchpad makes it really easy to provide your own custom packages, and even has a vast build farm to build packages automatically for different architectures and different Ubuntu releases. The only down side is it doesn't build rpm packages.. Thats fine by me, but that would be really useful for those wishing to deploy on a RedHat based distro.
He took the current package and pulled out a lot of the setup to make a "vanilla" installation. You can then look over at the PHP 5.3 PPA he's provided on his Launchpad account and grab this most up to date package.