Hey! Not true with my 5D. Is this a glitch in the firmware perhaps? I always delete images by formatting the card. It's simply quicker than 'deleting all'. Hhmm! Or maybe I made the mistake of forgetting to format a card at the time it was inserted and continued shooting with a fresh card that was half full of images from another camera. No, that can't be it. I might forget to format a card once or twice, which would temporarily upset the numbering sequence, but subsequent formatting would soon bring everything back into line, wouldn't it?
I've never had any issues when sharing a CF card between 10Ds, 20Ds or my 300D as long as I remembered to format the card first thing after putting it into a camera. I don't have 5D so I can't investigate myself... maybe it is a bug.
Hmm.. It doesn't seem that slow to me, but I've never used a card reader. I think I'll stick with my current workflow for now.
The 20D is USB 2.0 so it wouldn't be as bad as other USB 1.1 cameras but a card reader is certainly faster. Especially if you switch to a FireWire reader.