As he is talking about the limit on Apple.com I am assuming it's the current shipping Intel iMac.
The answer as far as I am aware is currently 2Gb.
In the past Apple have been conservative about the max memory limits as memory was not shipping in high enough density DIMMS for the real upper limit to be reached.
The max RAM than an Intel iMac could ever be able to handle is defined by the chipset it uses, in this case an Intel 945. The upper limit on the 945 chipset appears to be 4GB according to Intel (which is the upper limit for a 32bit CPU anyway). This may mean that when 2Gb DDR-2 SO-DIMMS are available in the appropriate speed that the current iMacs can take 4Gb of RAM. It is possible that Apple have implemented a 2Gb limit in their firmware though.