I don't know either. But I think both Windows Phone and iOS have some sort of central notification server, so I assume that even if Whatsapp needs a keep-alive every second (or 5 or whatever), it is the server that responds to that. Whenever the client needs anything, the server can wake him up (using a long-lived connection, probably with a wake-up every 10min or so to prevent NAT from killing the connection).

In any case, iDevices, Androids and Windows Phones probably have a much shorter battery life than the N900 (don't have experience with any of those, but friends report charging every night..), so the effect is not so significant..

In any case, Scorpius, you've done what you had to do (read: what people here wanted you to do! and your new implementation works flawlessly, so no complaints here