I've noticed (quite a long time ago) that Dataplan Monitor shows excessive traffic usage. I was wondering what application is responsible for that.. as nethogs app is available now, i tracked it down. Turned out to be a nasty bug:

RSS reader DOES NOT honor "update via WiFi" choice in the settings

Actually it performs full background automatic update even while connected via GPRS. Unlimited data plan is not available for my contract, so it costs me - ~10Mb per $.32 = $3 a time. Do that 3 times a day when no wifi is nearby and $10 is gone. Let that happen four times a month - and there is $40. Totalling ~$150 for 3.5 months i own n900. Sounds no good? No one will compensate me that.