Having spent some time in my small studio today and using a Canon 50mm 1.8 Mk2 on both the 1dsmk1 and 1dsmk3 and resizing the mk3 files down to mk1 size I find that there really isn`t a huge difference in the converted files. Slightly different colour balance out of the camera but still very close( I included a good white and clicking on WB in ACR brought the files close enough that only viewing them together on the monitor can separate them.
The 50mm lens is probably a good balance, maybe not good enough to bring out the very best of the mk3 but good enough to stretch the mk1 to it`s limit.
I`m still seeing from as close as I can get the exposures a very slight DR advantage with the mk1 (more so in shadow detail) but nothing what so ever that the very slightest change in exposure wouldn`t put right.
So! what are my conclusions?
If you want a 1series FF Canon, are on a budget and have a decent array of reasonable lenses with some L`s and can put up with the older functions of the mk1, like slow write speed, small dim lcd, much delving into the menu system to make changes ect. you will how ever get basically a bullet proof camera that will give years of reliable performance and give excellent results with the average Canon lens (even the humble 28-135 looks pretty good when shot on the mk1) I, myself have a sigma 20 1.8 that really comes to life on the mk1 (maybe not quite up to the Contax 21mm but very satisfying none the less.) My 24-105 and 17-40 do a stellar job on the mk1. My Contax 35-70 is good (as you would expect) but doesn`t shine much above those previously mentioned Canon zooms. I watched as a mint 1dsmk1 went for around £800 on E bay last week and that is quite a bargain when you consider they were £6,300 when first launched.

The 1dsmk3 is an absolutely super camera that has given the pro shooter the means to present large images that hold up very well when printed, superb feature set , battery life, write speeds etc etc. but it does need lenses that IMHO Canon doesn`t make as yet( more so down from standard to WA) It`s still an expensive camera that due to Nikon`s catch up has dropped down in price quite a lot but in reality is still a mortgage job for lots of people. If you can afford one, are prepared to spend money on alternative lenses and use MF you will land up with a camera that can do almost anything you want with headroom to crop and still land up with enough res to print big.

That leaves us with the middle runner, the 1dsmk2. The best compromise in the Canon 1 series FF cameras at the moment as far as I`m concerned. Enough res to allow some cropping, enough fine detail when using Canon L zoom like the 17-40/24-105 and good performance for most decent lenses, good battery life and write times and file sizes that won`t eat away at your hard drive space too much.
I`ve never had an image refused from stock work from the mk1 or mk2 other than the occasional missed dust spot. My refusal problems started with the mk3 but I now realise that a touch of sensible sharpening does get them through.
Amazing really because last week i shot a few images on a G9 compact that had decent impact so I upsized them and they got through IQ I`ve also had 40D and 450D images sail through. (it does make me wonder if those big 60Mb natural sized files are getting scrutinised more closely)
I don`t see many 1dsmk2 cameras for sale so obviously other people have realised the value of them.
My mk3 went into canon service twice(once for viefinder alignment and once to have the AF sorted) now I have micro adjusted my lenses I think I`m getting pretty good AF but seeing as I use mainly MF lenses on it then it`s not a problem. BTW the 70-200 F4 IS is great on the mk3.

I suppose that most regular users of this forum already knows much of the above but we do get browsers looking to purchase so it might help someone.
I`m not the most scientific tester but one thing I always do is to say it as i see it in my images.