I took my iMac to an Apple Store in France for a mainboard replacement. I won't say where, but when I actually saw it, the glass was chipped quite badly at the base, where the metal touches the glass. So it does happen. Had a tense conversation with the manager and they replaced the screen/glass.

Alas a month later apparent RAM problems (beep beep beep, machine refusing to go oo sporadically). They then changed the ram, saying it was unrelated.. 3 weeks later the ram issue resumed... AppleCare, to their credit, almost immediately then offered to replace the whole machine which i didn't expect but it's a relief. Im sure my iMac situation was unusual, but things dont always go as planned when a problem starts and they start ungluing these machines and moving bits about!