Padraic Brady has a quick post in response to some questions he saw about installing PHP on a Ubuntu system (9.10/Karmic Koala) via apt-get.

As there was some exclaimation marks on Twitter about this, here's a quick article on installing PHP 5.3.1 on Ubuntu 9.10 using aptitude or apt-get. I use aptitude primarily, but the installation uses normal .deb files. Since Ubuntu will not officially adopt PHP 5.3 until next April, the PHP 5.3 debs I use are pulled from the Dotdeb Debian "Lenny" repositories. These are perfectly compatible with Ubtuntu 9.10 and only require the manual installation of two extra dependencies which Ubuntu cannot resolve itself.

You'll need to add in some new sources to your list, but after that it's a pretty simple process to pull in the PHP 5.3 packages, add in a few extra libraries and then run the update for apt-get to install it all.