Interesting thread. Glad that I stumbled across it. My 2008 iMac has streaks in the upper-left corner that appeared and grown over the past year or so. At first I thought it was dust or candle smoke that got vacuumed in from the bottom, and I was planning to research whether it's possible to pull the glass and clean it.
Despite the good experiences I've had with this iMac, my next desktop probably will be a Mini. One reason is because I want a really big display. Another is because the iMac has gotten so thin in the latest generation that I'm concerned about thermal failure becoming more likely.