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Hi Baz,
I can't comment on Canons as I own a Nikon D3s, but it is exceptional at higher ISO. That is the main reason I bought it. Basically I reckon ISO 64000 is roughly the same as old grainy 1600 ASA film! It goes up to 102,000 bit there's way too much noise.
From my understanding from what I've read the 5DMKII is almost as good ISO wise as the D3s, so using 12800 ISO (stated as its standard and then can be pushed higher) you would still get quality images. I recently used ISO 8000 on some wide field shots and couldn't believe the Milky Way detail and fine grain. You could easily use ISO 3200+ to get what you want I'm sure...
Below is a shot of two 30 sec exposures at ISO 8000 stitched together to make a panorama. I was amazed at how well they stitched too! So depending what you want to do, if it's shots like this you're after you could make 2 or 3 quick shots in a row and stitch them, making sure the stars haven't moved too much by the end of the exposures, thus being able to use a longer focal length lens...
Wayne
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