I find it a bit iffy how some people talk about backwards compatibility, and recommending still supporting browsers such as NS4 as a way of making every visitor count.
Going off on a bit of a tangent, what about people with various impairments?
By sticking with tables, and supporting NS4, you're generally making it a lot harder to look after those visitors. If every visitor counts, then that means them as well. CSS based designs can be much more accessible (not necessarily, but they can be).
So how about the TENS OF MILLIONS of users worldwide who don't use NS4, but are given a rough ride because a designer would prefer to support the <1% of NS4 users, compared to the realtively higher percentage of impaired users.
Linking back to the topic about CSS and SEO. SE Bots are a little like visitors with visual impairments. Bots can't 'see' the page, and well structured code should generally be easier to read and sort out than stuff that is all over the place.
CSS based designs generally enable more semantic markup(again, depends on the code produced), which in theory should be easier for them to read and parse. Though I've seen no real proof of that, it does make sense, and helps in all sorts of other ways.
If you're going to claim every user counts, mean it, don't just look at the stats in your logs, think about the people using those browsers as well.