I think the Fantastico developers like to see that new versions of software and scripts pass their tests for stability and quality and see if there are any major problems before they are added into Fantastico. Unless you need something specifically from Joomla! 1.5 then I would recommend just waiting till your Fantastico control panel says that there is an update available and then you can update automatically (especially if you don't have a lot of knowledge regarding installing scripts manually).
