When the next version of Ubuntu is released on October 29th, PHP developers won’t be able to upgrade to PHP 5.3 through the included package management 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.
It is unlikely that Ubuntu will see official PHP 5.3 support until April, 2010, based on the history of Ubuntu release cycles. However, you can still compile PHP 5.3 on Ubuntu yourself.