There's a disturbing rumor floating around the Mac web today. According to this article at Infinite Loop, Leopard will no longer support Input Manager hacks. Input Managers allow programmers to insert code into cocoa applications, thus allowing the addition of new functionality. etc. These are somewhat controversial as some claim this is a potential security hole. Nonetheless, Jon Hicks brings this up in connection to my absolute favorite input manager plugin, Saft for Safari, which will presumably be rendered non-functional in Leopard. As Hicks notes, its not clear yet whether this also affect the widely used SIMBL plugin architecture (whose author, Pith Helmet developer Mike Solomon, says he won't be sure until he can play with Leopard). If, like me, you're a big fan of Saft and SafariStand and Chax, etc. this is cause for concern.
It is to be hoped, of course, that the authors of the various plugins will find some other way of implementing their feature enhancements, even if Apple does close the Input Manager route. The best thing would be for Apple to implement an open plugin architecture for doing this sort of enhancement, but I won't be holding my breath on that one. In any case, this is a potential concern for those of us sure to upgrade to Leopard right after it launches.