All 2011 iMacs will support 32GB unofficially except for the late 2011 iMac "Core i3" 3.1 21.5-Inch (MC978LL/A - iMac12,1 - A1311 - 2496) which appears to be capped by EFI at 8GB.
You can check out your specific iMac by finding it on this list, then checking out the RAM section:
Also, here's a list of the unofficial max ram support for every Apple computer since the G3:
Note that Apple may update the EFI at any time and enforce it's stated maximum RAM, and of course Apple won't support going above the maximum RAM.
Also keep in mind that you have to match Apple's memory timing specifications for new memory very carefully. I found with my Macbook Pro that getting "close enough" still resulted in an occasionally flaky system, and I had to not only get the correct memory speed, but the same timing for it to work. That said, it does support twice as much memory as Apple states, and many people do find success adding more memory than apple recommends.