So, that's what I've done, you can do the same here http://www.apple.com/support/itunes/contact/ . Follow on screen instructions and wait for an answer...
I've just tried to do this but it appears to get any further then the page after you will need your serial number. Luckily you can click the 'My Products' link and assuming you've registered your mac in the past the serial number will be available there.
Since I'm out of applecare though I'm just crossing my fingers that they respond. :/
I imagine that you have updated the serial number but... how did you do it?
It could be a serial number thing but I highly doubt Apple would start checking them for services like this.
If there was a problem there should be an option to enter it manually.
They haven't done so in the past you are correct. But given that they've applied a 3 activation hardware limit to iCloud I guess they chose the only thing that is unique to each piece of hardware.