@arzleb: I'm not sure what to say, but one thing I can tell you is that Maemo will never forcibly or automatically install updates without your say-so. Nokia's attitude (which is good) is essentially "you own the darn phone - you do what you want with it", and so they don't meddle with things like that. There is an app that adds a "Tablet Mode"/"Phone Mode" button to the power button menu, which would allow you to switch off phone functionality temporarily, if you wanted to make sure it didn't use data. Other than that, I can't help you, as I don't have a cell plan on my N900.