While the support article titled Mac OS X 10.5: "Blue screen" appears after installing Leopard and restarting doesn't identify Unsanity by name, it does recommend checking the Leopard compatibility of "any application 'enhancement' software." Application Enhancer (APE) is the system add-on, produced by Unsanity, that modifies the behavior of Mac OS X. The support document goes on to detail the steps needed to remove APE from the system via the Terminal.

Unsanity's blog claims that the problems experienced in upgrading Tiger to Leopard with Application Enhancer installed are isolated to cases where older versions of APE were in play. Says the post...

I'd like to note that the problems some people were experiencing while doing the Mac OS X upgrade and being unable to boot due to Application Enhancer installed may have been caused by the "old" versions of Application Enhancer not properly recognizing the system version. The latest 2.0.3, released on 13 March, is properly detecting the upgrade and refuses to load so you should not have any issues with installing Leopard over it. For this reason, we recommend you upgrading to 2.0.3 before upgrading your Mac OS X. It will properly detect the system version and deactivate itself.

Anyone who has called AppleCare for system support on a machine running Application Enhancer can likely vouch for the fact that Apple has never supported its use. If you are using APE and plan to upgrade to Leopard, it is our advice that you uninstall Application Enhancer beforehand.