Beats me. I suspect Canon won't be selling very many 1Ds MkIII bodies once the 5D MkII is available.
The 1D series is built better, with improved weather sealing. The AF system will, I think, AF at f8 rather than f5.6. It has a slightly faster continuous shooting speed and a wider range of exposure compensation.
Howver the 1Ds MkIII is bigger, heavier and much, much more expensive. I suspect that Canon have a 1Ds MkIV up their sleve which will "outfeature" the 5D MkII with a higher pixel count and even more bells and whistles. If I was them, I'd release it ASAP, possibly at PMA in the spring of 2009.
for more of my thoughts on this topic and a comparison of features.