Aaron, the model by which a distro's packages update isn't exactly compatible with the typical web shop's fast advance forward.
Because the distribution seeks to integrate things to a point of extreme stability, typically we won't actually update a stable release like 10.04 to the latest upstream, as this may cause incompatibility with the dependent software shipped with Ubuntu.
Instead generally we will cherrypick patches and fixes from the newer versions and push them into the stable releases as "StableReleaseUpdates". This reduces the chances for regression while still solving the issue of fixing bugs.
Here is a list of the fixes that have been released, or are accepted as needing to be fixed in the lucid php5 package:
Also, there is a process by which we can backport newer upstream versions into the stable releases, but it is typically at a lower priority than bug fixes and work on the next development release. However, if users follow this process it shouldn't be too difficult. We'd really love to have more people requesting/working on backports!
(On a more specific note, PHP 5.3.3 is in 10.10, and 5.3.4 should be in natty (the future 11.04) unless it introduces any horrendous regressions).