As Johannes Schluter mentions, the results of some benchmarking have been posted concerning the performance of PHP 5.3 versus the current 5.2 series:

Dmitry posted results of performance test comparing PHP 5.2 and 5.3 to internals which are impressive numbers.

The improvements were measured based on several popular pieces of software like Drupal, typo3 and WordPress. The overall performance gian was around thirty percent across the board.