Not sure if the new Canon 24mm f/2.8 IS is worth waiting or not. It is somewhat appealing - small, lightweight, USM AF, probably pretty sharp, and IS is always convenient - but the price is really quite high for what it is. I know a big part of that is just the exchange rates finally catching up, but still, it's pretty hard to see a US$350 lens replaced in the lineup with a US$850 lens.
If a full manual lens and size are not major concerns for you, I'd recommend checking out the samyang/rokinon 24mm f/1.4. B&H currently has it for pre-order at $700. I have the rokinon 35mm f/1.4 and it's an exceptional optic regardless of price (once stopped down to f/2.8 and beyond). I've also used the 24L II and it is a very nice lens - great if you need AF, but there are better options out there for pure IQ (24mm TSE f/3.5L II, the 21mm Zeiss you mentioned).
vuilang makes a good point - foregoing a new lens and getting a full-frame body instead is another option to consider.