Mr Chin from QCD just came down to replace my iMac's hinge. A very friendly and skilled guy. iMac is back to normal now. Seems like this problem is quite common. Apple really needs to improve on their QC.
Reviving an old thread in case anyone faces the same issue and is unable to get help from QCD or A.Labs. QCD and A.Labs are Apple Authorised Service Providers and usually only service in warranty products for free. For out of warranty products, it is near impossible to push for a replacement or repair for free, even if it is a widely documented issue that Apple knows about, but has not issued any statement for a free warranty extension.
In Apple stores overseas, where repairs are done by Apple themselves in stores that have Genius Counters, issues such as this iMac 27" hinge issue might be fixed for free. In Singapore the Apple support line is manned by Australians (not all the time). I've had experience with them, and they sometimes issue support tickets for customers to get repairs for free at Apple Authorised Service Providers.
This iMac issue is documented at my blog if anyone if interested. The issue is caused by the breakage of a plastic spacer that keeps the hinge in place. I have no idea why Apple chose to use a plastic spacer instead of a metal part.
I wrote an lengthy post in my
Musings About Life blog that I used to update infrequently sometimes about Mac and Apple stuff.