My main camera for event work is a 1DmkII usually paired with a 70-200 f/2.8, and I supplement this with a 5D paired with a 24-70 f/2.8.
1DmkII is a wonderful camera; size may be an issue for some, but I find the weight to be a feature rather than a hindrance (and if you put a battery grip on a 5D/7D/50D, the weight is about the same anyway).
The 1DmkII takes some time to get to know, but once you "get it", it all makes sense and the camera is a breeze to operate. The weather sealing is nice, but not something I really need all that much. It's also great to have useable outer focus points as well as dual card slots for redundancy.
That said, for landscapes, the 1DmkII would not be my first choice. Its 8.2MP is quite low for that type of shooting, where it is outresolved by a more modern camera. Also, for landscapes, you're either going to be manually focusing (live view or otherwise) or using the centre point, so the AF performance of the camera is less of an issue.
If you were buying a "new" camera, I might suggest a 1Ds mkII; its 16MP will be a small step up from the 5D, and you get all the other great 1-series features to boot (and it can now be had for <$2000, which is an absolute steal). But since you've already got the 5D, I'd say keep that until you can afford a nice big step up to the 5DmkII.