Lots of people have reported this problem. It's the ambient light sensor, which responds even to a shadow from a hand holding the device on the left. There's a time delay of a few seconds before the display brightness adjusts.

You can see this for yourself. When you notice the display dimming, take your hand away or rotate the N900 towards a bright window. Within five seconds the display will return to its previous brightness.

There are two workarounds. The first one is to cultivate a habit of holding the device differently. The second workaround is to set the screen brightness to full (under "Settings" "Display"). At full brightness the screen does not vary in response to the ambient light sensor.

Regards,
Roger