Kristof, Did you get an answer to your question? I've been working on a multi-player application recently as well, though I've chosen to host the server (originally). I'm now reconsidering my choice, though, but the library I'm using fully supports peer to peer communications. The underlying protocol is built on top of Google Protobuf. It's essentially a full duplex RPC stack built on top of Netty, which can use Protobuf. Here's the URL to the RPC protobuf stack: http://code.google.com/p/protobuf-rpc-pro/
The author has been very helpful and I've found a couple of bugs, nothing major. I also had very little issues getting these libraries working on my Android phone, but they're not terribly "compact." Nothing extraordinarily large, just not small :). So far, I've had no issues getting the communications working both synchronously and asynchronously. As such, I may be moving my game over to a "peer to peer" style, and just provide the necessary location/registration server that would be used to find existing games/server.