I figured if I created a thread about porting WhatsApp to the N900, and what I've discovered so far, others could share their knowledge or decided to take a look.

I'm studying for an exam that's in a week or so, but I fired up WireShark, got my trusty Nokia E66 connected to WiFi (with no SIM card, to force WhatsApp to use WiFi) and started logging packets.

Lo and behold, what I got in return cheered me up. It appeared to be nothing other than bog standard XMPP! Maybe, just maybe, it won't be too much trouble to port it over. They might have some proprietary extensions to lock other clients out, but I can't really recall. As I said, exam > WhatsApp