Far more useful would be the percentage of IP addresses which have only ever attempted to access the site using a compatible browser.
FFF,
With my limited knowledge, I don't think that would work. Your average member of Joe Public (e.g. me) gets assigned a different IP address everytime they log onto the internet. I could log on one day and use IE5 or 6.0 and log on the next day with Netscape Navigator or something else. I would be the same user with a different IP address in each session.
FWyg.
(edited for a few tarrible spilling mistooks)