Nice site!
Currently tracking the version in a local database and checking for changes hourly via cron. I've half implemented checking github repo's but its not 100% accurate yet. Not sure how you are doing it but I am going to focus on an update being any change to a branch. So its basically tracking branches only.
Well that should be possible too. However if you add a package later, it would be easier to edit your own package file then to log back in to updateditis and re-upload a package file. Automatic for the people
I have almost finished the ability to add custom projects (within reason). Perhaps I could then implement a service which auto-detects packages. So, user Bob creates a repo in github. He adds a file in the root called packages.json which contains a list of the packages he is using. He then adds his repo to updateditis.com. Updateditis periodically checks his packages.json file and if it detects a change in one of the packages versions, it emails Bob saying "For project XYZ, the package ABC has been updated to version 1.2.3, you should consider updating it."
No plans for an api as such yet. Although I might add in the ability to change the notification type. Options would be a push url, sms, jabber notification and anything else that might be useful.