Google provides a variety of 'cards' for Google Now (http://www.google.com/landing/now/). Is it possible to create your own cards? The system looks pretty modular, but I haven't found any documentation or instructions to do so. (I believe you need to supply the content of the card, and some way of signaling when it is supposed to be shown. There is probably just some interface that you have to implement.)
If there is no documented solution, a hackish/undocumented way would be ok, too. I'm mostly curious how it works.
Edit: Specifically, does somebody have knowledge about the internals of Google Now, e.g. by decompiling the .apk? What I've seen suggests it is pretty modular, and it should be fairly easy to drop another class into the .apk, or to maybe inject code using Cydia Substrate. I know that there is (as of Nov. 2013) no official way to add new cards.
Actually Google announced last week that developers can now develop custom Google Now cards:
http://www.google.com/landing/now/integrations.html
However, a developer guide seems not available yet.
Edit: On the end of the page they point out that:
We'll let you know when we are able to onboard more partners