Just this morning, one of my buddies was ragging on me that TextMate was cool because of snippet expansion. I personally think this is optimizing the wrong thing since the typing part of programming is the easy part. Nonetheless, I'm happy to see that Vim has a knockoff. Best of all, it's easy to use and pretty useful.
You can get the plugin here. Once you install it per the instructions, you can open up a Python file, insert the text "def", hit tab and get what's shown in the image. Hitting tab again jumps between the fields. Even better, there's a snippets file for Genshi.