Jaiku is becoming JaikuEngine

You can examine several AppEngine code bases on github.com. There's something to be learned from all of them.

Shameless plug: not that it's at the same scale, but I put together the game of Go as an AppEngine app. You can play it at http://go.davepeck.org/ or read the AGPLv3 sources at http://github.com/davepeck/appengine-go/

(I wrote "Go" to share with the audience of a talk I gave about scaling with AppEngine; I do a lot of AppEngine and AWS work for my clients but alas can't share that work.)

Me too. Do you (or others) also want to build something on top of it?

I'm not very familiar with either code base but the fundamental differences are that Jaiku is built to scale whilst Laconica is built for federation between different servers.