I think, have a feeling, when charging with the dedicated charger, it won't stay at 500 Mhz, but when charging at the same time when USB-data connection is established (but idle) it runs @500 Mhz even if there is nothing busy and the load is 0.00 When charging whole night from PC USB, even after it had fully charged long ago, the N900 will be quite warm if not hot in the morning. But if it has been connected to dedicated charger, it is cool in the morning. So there seems to be a difference and it seems like N900 do not idle at all when charged from "real" USB connection.