Here's another Assassins! derivative, albeit one a tad bit dirtier and less "your application ready": the boringly named textapp. This is the guts to my all-inclusive text messaging dispatch for App Engine. It has handlers to accept both Facebook's SMS requests and App Engine's XMPP-based IM requests.
Because I was feeling too lazy to set up something friendlier (to me) like "darcsit" or my own django+darcs solution, I've posted the current version of textapp.py on github's gist. It depends on my webappfb.py.
I think that as it stands right now textapp cross-cuts too many concerns to be immediately appropriate to any project I might donate it, and it isn't "big enough" to merit its own project right now. However, I do think it might be useful for interested people.
First of all, it is a bit dirty: you'll notice that it makes reference to my domain model named Player. The only important detail for textapp is that this model does my Facebook uid to XMPP jid mapping (with some authorization magic, of course). There is also no localization, yet. (I've been wondering for a few weeks now how soon I should start on Assassins' l10n plans, partly because I want support for Pirate, which is my preferred Facebook language.)
High level there are three important parts: RegEx-based dispatcher, Message classes, and request handlers.
The request handlers you simply provide a callback (as application) and plug into the appropriate places: your main handler (/, most likely) for FacebookSmsHandler and the App Engine XMPP handler (/_ah/xmpp/message/chat/) FacebookXmppHandler. You'll also want to handle the canvas() function for FacebookSmsHandler, as that does double as your application's "front page". These are normal webapp request handlers and should be wired accordingly.
The message classes are populated by the above handlers: FacebookSmsMessage and FacebookXmppMessage. Both message classes provide the same interface and for the most part handlers don't need to worry which one is which. (Both are based on App Engine's xmpp.Message class.)
The dispatcher TextApplication is a simple RegEx to handler mapping class that should be familiar to most Python web framework users. RegExes are tried in listed order, attempting to parse the message body. The message and passing RegEx groups as keyword arguments or positional arguments (depending on whether or not your RegEx uses named groups). Handlers can be be functions or classes in the usual Pythonic "callback" fashion.