Fluid templates are by default cached as PHP code, starting with TYPO3 4.6.
You will find all background informations on this on the blog of the developer of the caching, Sebastian Kurfürst: http://sandstorm-media.de/blog/2011/07/31/fluid-static-php-caching.html
The official announcement (if it can be called like this) is in the release notes of TYPO3 4.6.
Apart from the code of the complier, I'm not aware of any other technical background information.