I agree with Alex. G&C has a huge problem with nested tables everywhere, and we just started cleaning up the old dinosaur – not a small task with over 6000 pages.
Pages without tables CONVERT better too – the number of visitors who leave because of slower loading is high enough to make a difference there.
So it’s not simply a matter of SEO, it affects your conversions directly.
That said, we are running into some display problems – we do use some collumn/row type things and they can be problematic.
I am currently debating whether we need to change the entire look of the layout, or whether we should allow one table per page.
There is a thin line sometimes between better SEO and better visitor results. Taking a site that sports a huge number of return visits and changing the look and feel will alienate some loyal visitors. So – we are looking for a compromise.