The Zeiss 21mm ZE will shortly be in the shops!
I have just looked at diglloyds sample shots taken in NY in his review and they are amazing. Even at f2.8 the corners look very sharp and the contrast very high (at 21mp). He says that the lens is good for 30-40MP.
This is good news for Canon users !
However I was less impressed with the 18mm ZE.