When you use tabs, Safari sets the tab's title to include the domain name as well as the page title. For instance, macosxhints.com - Submit a Hint. However, when you have more than one or two tabs opened on the same site, Safari drops the domain name from all the tabs, and just includes the page title: Submit a Hint
It somehow doesn't seem to be completely consistent, but Safari definitely changes the tab's titles on the fly, so as to avoid many macosxhi... tabs, which would be basically useless.
[robg adds: True, there's not really a hint here, but it's this kind of attention to detail that makes much Mac software so pleasant to use. I had never noticed the behavior in Safari before, but I had noticed that its tabs seemed somehow 'cleaner' than those in Camino ... and now I know why!]
Mac OS X Hints