Jakob Nielsen has a good post about how the Yahoo Finance site uses tabs (the site uses tabs in a way that Nielsen thinks is good). There are some good points to consider if you are about to try a tabbed design.
One comment startled me though - Nielsen approves of the Yahoo Finance site in part because:
"It uses tabs to alternate between views within the same context (not to navigate to different areas — a common mistake introduced by Amazon.com)."
I do wonder, however, whether many people are confused nowadays by Amazon-style tabs? The "mistake" of using tabs to navigate different areas may have become so widespread that it is no longer a mistake, but a new convention.