What is the PHP 5.2 dependency?

There is no one specific thing. Rather, the module is written assuming PHP 5.2 as the baseline. It may or may not work on PHP 5.1.6 and earlier versions (I suspect it would generally work with 5.1.x, most likely), but I do not intend to test or explicitly support such earlier PHP versions. That said, I would be willing to fix any (very) minor issues preventing it from working with 5.1.x, provided somebody posted a patch for them, so do report back if you give it a try.