I was trying to run my WhatKeys app at work, because I actually had a use for it, and discovered it doesn't compile on Leopard. I'll fix that when I get a chance.
On a related note, the developer of the ShiftIt app I mentioned has a blog called "Cocoa Tips and Tricks." One of his posts is about implementing hotkeys, and one of the comments to that post mentions that Snow Leopard has an NSEvent method that makes it real easy. I see from the doc that:
Key-related events may only be monitored if accessibility is enabled or if your application is trusted for accessibility access (see AXIsProcessTrusted).
Note that your handler will not be called for events that are sent to your own application.
I'll have to check whether the DDHotKey library I used has these limitations. Regardless, this is nice to know.