Probably your best bet is to use the PHP5.3 packages in Debian Experimental.
http://packages.debian.org/source/experimental/php5
I can not tell you how different the Debian and Ubuntu packages are, but they should work reasonably well. You might need to recompile them for your Ubuntu release if various libraries require different versions. These packages are what will appear, in some form or another, in Ubuntu 10.04.
You're totally on your own with them though. If your system breaks, you get to keep both bits. You might consider following the mailing list at pkg-php-maint@lists.alioth.debian.org
Zend, who are the commercial entity behind PHP, have a set of yum/apt repositories that you can use to install and update PHP 5.3 on both Ubuntu and RHEL/CentOS.
There is a community edition that can be used however you like at no cost, although not all components are open source ("free as in beer" only).
To install you would add a Zend.repo file to /etc/yum.repos.d (or edit /apt/sources.list) and then do
yum install zend-server-ce-php-5.3
or
apt-get update
apt-get install zend-server-ce-php-5.3
Full instructions are at Choosing Which Distribution to Install
It includes an opcode cache (Zend Optimizer+) and a debugger (Zend Debugger) although these are perhaps not as widely used as APC or Xdebug, possibly because they are not open source.
It also includes Zend Framework and various extensions such as the php-java bridge, database drivers (not all of which are included in the normal PHP distributions) and a web-based management console.
There is also a commercial version which includes support and additional features.
Best Answer
I've had success with these packages :)
Hope it helps!