Support non PSR-0 packages
Composers autoloader supports non PSR-0 packages if they follow the so called "PEAR-style" (see http://getcomposer.org/doc/04-schema.md#psr-0).
A lot of packages on packagist.org still don't follow the PSR-0 standard. The only way to get those working with Flow, is to include the composer autoloader manually. I did this by adding following line to the Package.php of my distributions main package:
require \TYPO3\Flow\Utility\Files::concatenatePaths(array(FLOW_PATH_PACKAGES, 'Libraries/autoload.php'));
But obviously this is just a hacky work around with unpredictable side effects