I'm receiving +10,000 unique visitors daily and aprox. 85% are from search engines. Homepage is PR5, but most traffic are for inner pages (thousands of different pages) where PR is usually PR3, PR2 or PR1.
Yes, you're right. I'm getting many visitors because I'm ranked on Google #10 for many many different keywords.
One PR5 site I do consulting for gets 1500 uniques per day and makes $5 million per year so its not all about the traffic or PR.