I have a 5D and a 1Ds mark 111. Both are excellent cameras. We use them mostly for weddings etc, but also sometimes for personal work like landscapes.
Both are good at higher ISO settings. You say that you are interested in a new camera for landscape work, then say that maybe you will wait for a new 5D because it might have better ISO capability. Of course you may need a high ISO for some types of photography, but most likely not landscapes.
I could justify buying a 1Ds because it earns my money and the old 1Ds mark1 has done 4 years and maybe 150,000 pictures without fault. However, the 5D would have done ok. I just wanted a 1Ds!
The extra pixels are nice but probably not necessary unless you are making very big prints.
A new 5D may well come out soon, but the current one is really good, and if you never bought another camera you probably would not be disappointed with the pictures it produces.
On the other hand... The 1Ds has got a better viewfinder, has live-view, and the battery never seems to need recharging. Oh yes, it also has two card slots for more capacity or in case you are paranoid. It is also lighter than the old 1Ds, but a lot heavier than a 5D.
Then again, you can buy four 5D cameras for the price of a 1Ds mark 111.
Seriously. You will not be sorry to buy a 5D unless you just always have to have the latest.
Jim