Latest Version for PHP 5.3 on Ubuntu is just PHP 5.3.3. This is not so good, if you want to have some of the plenty bugfixes which will be shipped with a later version.
If you want to upgrade to 5.3.5 (5.3.6 support may come later), you can use this inofficial ppa by the nginx user at launchpad:
For ubuntu maverick just create a file:
$ gedit /etc/apt/sources.list.d/php5-ppa.list
with the contents
deb http://ppa.launchpad.net/nginx/php5/ubuntu maverick main deb-src http://ppa.launchpad.net/nginx/php5/ubuntu maverick main
$ sudo apt-get update $ sudo apt-get dist-upgrade
Now you have php-5.3.5 installed on your ubuntu box! Have fun.