As you can see, there are concentrations at each end of the spectrum with pockets for each version. This is to be expected. This is not a comprehensive survey, but it is interesting to see what the folks most interested in getting things done with Notes are using. Fortunately, we have coded eProductivity to support versions 6.02 - 8.5x of Lotus Notes across all platforms (yes, even Mac and Linux).
I plan to update this over time to see what more we can learn.