As pelago notes, I don't believe that's the case.

I have plugged my N900 in overnight when it was fairly full (80%+ charged). The next morning (8 hours later), it comes off the charger at ~94%. Given that it discharges around 5% per hour (for me). So give it 2 hours to charge completely, that leaves 6 hours on the charger at 5% discharge rate per hour, it should come off the charger at around 65% or less. This has never happened!

Has this ever happened to you? Have you ever taken your N900 off the charger with less battery charge then when you put it on the charger? If so, I suggest you send your N900 in for service. *edit* I see it has, I stand by my recommendation that you have your N900 serviced (or at least verify you don't have any programs running at 100% while charging).