Interesting. I'd have never thought that this would be the case - I guess it's similar to odd viewing percentages in photoshop (as a very rough analagy?!) being jaggy?

There is a setting in my 40D to not allow selection of these 'in-between' ISO settings and I have that selected by default. IE - when I flick though the ISO I only get 100/200/400... as options.

I agree with kevin though - it's no doubt a marketing ploy to a certain extent.

Graham