Any thoughts on why this may be happening? it is quite annoying and strange...

it happened to me and i had no idea what was going on also. When you put your finger over te top left of the phone/top right in portait/ its a sensor that automatically changes the brightness.

Hrm i don't think it's that... the sensor isn't being covered...

doesnt necessarily needs to be covered. even if your finger is creating a shadow it can make the screen brightness adjust.

Yep. I operate the device left-handed (hold it in my right and use my left forefinger on the screen) and I have two issues: the screen brightness changes all the time because of the shadow from my finger / hand, and my right hand seems to absorb a lot of the cell signal (the cell antennas are on the right of the device).

The second one only becomes a real issue indoors or in poor coverage areas.

I guess the N900 was meant for right-handed people.

Yet pop-up menus are made for lefties... they appear under thumb for right handed people :/

as ArmandHammer said, it doesnt need to be covered... if im not mistaking the sensor works according to ambient light and therefore changes according to the slightest ambient change... also worth noting is that it's also a bit slow... in the sense that it takes a second or so b4 it notices the ambient light dimming or lit up b4 it changes the brightness accordingly... i think u can disable this feature by removing the power saving option in the display settings. (guys do correct me if im wrong on this one)

if you have a digital camera, you can do a simple field test without expensive light meters. just point your cameras lens to the directions your n900s top side is aligned when used.

then just press shutter button halfway and look at aperture and shutter time (f4 1/500 for example) when moving a camera little bit. specially when there is dark outside and lamps are used, simple 5 degree movement can more than double the amount of light.

just tested with my dslr. when facing a lamp(read: screen parallel to roof) : shutter time 1/5000. 5 deg turn and it changed to 1/1600 so amount of light changes 5000/1600 = 3,125 times.

Have exectly the same problem with both my N810 and my N900. It seems to me that there are certain thresholds for the ambient light sensor... So it can be that whenever the light in your room is around such a threshold but changes only slightly (maybe because you're movin the device a bit), the screen brightness will over-adjust noticeably.

And, as twisted said, it doesn't react immediately. It takes some time. I remember one day when I used my N810 indoors on a reletively sunny, but windy day, it would change brightness each time a cloud covered the sun... But it would become darker when the sun re-appeared behind the cloud and a little brighter again a few moments later. Funny thing was that I myself didn't even notice the change in ambient light, I only noticed the screen becoming darker and brighter.

The algorithm may need some tweaking.

This is the funniest thing that Ive ever heard!!!

I suggest you to destroy your display with a hammer to solve this "annoying strange" problem!

Are you kidding, aren't you!?