Jason Burby wrote a nice piece for ClickZ about the changing nature of web bahavior and analytics as tabbed browsers like Firefox (and soon to be released IE7) become the norm.  Jason identifies several aspects of analytics which are affected because of tabbed browsing, including the use of cookies, path analysis and time spent on site. 

While I'm not ashamed of being a Firefox-freak, I'm quite sure that I have wreaked havoc on web analytics specialists all around the world.  I typically open links in new tabs, and it's not uncommon for me to have 10-20 tabs open at a time.  Of course I also have to explain that type of behavior almost weekly when asked, "What's a good bounce rate?".