Update: I believe I've zeroed in on the bad egg - Logitech's Control Center 3.1. I just started having the problem on my new powerbook after installing it. Numerous complaints exist:

• http://www.macupdate.com/info.php/id/8154
• http://forums.macosxhints.com/showthread.php?t=105711
• http://logitech.lithium.com/t5/Keyboard-and-Keyboard-Mice/Tab-Display-bug-wit...

Safari 4.0.3 and Snow Leopard 10.6.1 seem to have a bug in tab display when the tab is loaded in the background. Bring them to focus and it fixes itself. Bring the tab to focus and it corrects the display.

Reported elsewhere too:
http://forums.macrumors.com/showthread.php?referrerid=59552&t=804431