I have installed Traffic Monitor to check how much data is transferred between my phone and the rest of the world. The traffic is categorized by application, but there is also the category "system traffic", to which all uncategorizable traffic belongs. Strangely enough, the "system traffic" consumes the lion share: 2 of 2.8 MB within several hours of idling (over wireless LAN).
Skype is off, only E-mail (K-9 + standard client), the news+weather widget and the browser are running. For these apps, a consumption of 0.8 MB is shown. Still a lot, but I can handle these. Apart from these: Traffic Monitor, Settings, Google Services (what's that for precisely?) and the Android keyboard.
What is responsible for the rather heavy traffic here? What are typical "traffic eaters" that could fall into the "system traffic" category? Is there a way to disallow communication while the phone is not used (blank display)?