How can I see what custom URI schemes are registered? One of the neat things about OmniFocus is that you can add tasks to it by calling omnifocus:///add?name={query} from a browser, or launcher (like Alfred or LaunchBar). I'd like to know what other functions there are to call for it, and other apps as well.

I poked through the info.plist of Package Contents of the app, but it only detailed that omnifocus:// was indeed registered as a custom scheme. And I couldn't find any reference to it on Omni's site as well.
I like to use RCDefaultApp to manage handlers for protocols and URIs.  There may be a better way to do it programmatically, but Rubicode's RCDefaultApp is very useful for a lot of other reasons too.

When you dump the launch services database does it show up there?  

 /System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -h