At work we have around 100 sites hosted for clients, some of which might not have been updated in a few years (I should point out these are sites we develop, so there's no chance a client's going to edit the site themselves). IE8 is going to be rolled out to Windows users with Automatic Updates enabled as of next week, so there's a small worry about auditing these sites in time.
When IE7 came out we had to spend the time going through each of them manually and checking everything was fine. This time around, although IE8 has a new rendering model, it's possible for the browser to render pages as if it was IE7. In general this has been hugely controversial, but for people in our situation it's pretty handy.
The easiest solution to having sites that may not work in the IE8 renderer is 'do nothing'. IE8 has a compatibility button that a user can press that renders the page as if it's IE7. If enough users press this button, a scary centralised Microsoft database marks you as a naughty site and from then on, IE8 users get to see you in 'compatibility mode' until some time in the future when you fix your site and manage to persuade Microsoft that you should be let back in to the halls of the worthy.
However that sounds like a mess, relies on users jumping through some hoops, and might be a bit tricky to get off the list at a later date. The strategy we've decided to go for is to explicitly mark all our sites as needing to be rendered in compatibility view, then turn this off for each site in turn as they're audited, at our leisure.