As Brandon Savage mentions in this recent post to his blog, PHP 5.3 (the most current release as of the time of this post) will not be included in the upcoming Ubuntu linux release and won't be upgradable via the normal package tools.

A meeting of the development team on July 30th nixed the inclusion of PHP 5.3 from inclusion in Karmic, the next iteration of Ubuntu for the desktop and the server. According to meeting minutes, there is concern amongst the Ubuntu security team that failure to include the suhosin patch in the PHP release would be a feature regression. Instead, the release will be referred to PPA until more testing can be completed.

Until the package is officially accepted and where users can update it as usual, you can still try your hand at compiling it manually and getting your Apache server up and running quickly.