I had written a Google app engine based application to create and manage bots that feed data into twitter. The application is similar to what twitterfeed does, with some additions. The applications is now up and running at http://twitteybot.appspot.com where you can register to activate feeds.
The data structure used is simple, but I am afraid it may not stand up to the scalability requirements. The code is hosted at http://code.google.com/p/twitteybot.
The persistence object is structured as User > List of Twitter Accounts > List of Feeds > List of Status Updates. Being a unidirectional owned relationship, when the status messages are fetched from the feed, they cannot be directly inserted into the Status Updates object without a reference to the user. Another problem with the status fetch cron job is that currently it requires a user name. This also has to be changed before the bot is fully functional. The twitter accounts also require username/passwords. These can be changed to support OAuth, but thats something for the future. The user interface is also plain HTML based, it has to be changed to make AJAX changes to the database. That should make the product more or less complete.
On the content side, there still needs to be work for getting content for bots. I was planning on bots for The Bhagwad Gita and Thirukurals. The Bhagwad Gita is on a webpage, not parsable by Yahoo mail. I would have to curl (download) it all to a text file and then upload the file. Watch out this space for updates and please do drop a comment if you would like to install the bot in your app engine account.
