since getting my N900 using the Data plan monitor ive noticed i've been using around 15mb a day, most of the day it's on wifi and not counted. But i was curious, to what the data actually was , i expected msn, google talk active data connection, rss and some other things, but i dont tend to sync email etc on 3G.

Anyway walking back from the train station i decided to run wireshark , which works really well, just needs a Meamo UI , which allowed me to see what data is going out on the Phone's network. I saved this to file and re-imported into desktop wireshark (which works perfect).

in say 10mins walking, my data plan monitor had increased by around 1mb. This is with RSS widget off and no browser open.
This is where it gets interesting, according to wireshark in that 8 min period with no browser open, i had downloaded
mynokiablog 175kb
ad.doubleclick 130kb (dont use adblock)
guardian 95kb you get the picture.
The thing is all the website's that where listed where websites i had visited in the morning, 10hours before.

So it looks like closing the browser in maemo you still get a connection to the various site's you have been to.
I would recommend d/l wireshark for your N900, the UI a bit cramp but it works really well and it's interesing to see whats going on behind the scenes.
I only had wireshark open in that smal period so it wasnt grabbing data from when i did have those sites open.