Tomlee59's post reflects one aspect of the matter, made to appear worse because a number of iMacs from (possibly) different environments have been concentrated into one arrival point. I don't contest the experience, because Tom has reported what he has found.
I have two Summer 2000 (bought new) and two Summer 2001 (bought near-new/unused) iMacs. They are always-on. They also have their rear feet elevated 18mm (0.75in) on timber, and 40mm in one case to keep its screen face vertical in a rack. Their tops are clear, or have no horizontal surface closer than 55mm above them. Their exit air is just perceptibly warmer than that from an actively ventilated Mac. Failure rate, other than the logic board of one new one three weeks after purchase, is nil. This is probably also an atypical experience. I don't know. Perhaps it illustrates that ventilation can work wonders.
de
I have two Summer 2000 (bought new) and two Summer 2001 (bought near-new/unused) iMacs. They are always-on. They also have their rear feet elevated 18mm (0.75in) on timber, and 40mm in one case to keep its screen face vertical in a rack. Their tops are clear, or have no horizontal surface closer than 55mm above them. Their exit air is just perceptibly warmer than that from an actively ventilated Mac. Failure rate, other than the logic board of one new one three weeks after purchase, is nil. This is probably also an atypical experience. I don't know. Perhaps it illustrates that ventilation can work wonders.
de