Probably not cold weather...cold weather will cause a "laggyness" or "motion blur" effect on an LCD that should go away at normal temperatures.

The N900 seems to have a lot of hardware faults. Weak USB port, varying battery quality, varying CPU quality, memory chips that are easily disloged if it's dropped, and now bright spots...I gotta say you'd expect better quality for something so expensive. It's not a huge issue for power users and tinkerers but these issues would terrify an "Average Joe."