Lots...LOTS of my clients use Gmail so this is kinda related to working with FCS since clients and I tend to email one another often during the course of a project. And sometimes clients are exactly friendly towards one another, especially when they're from competing agencies.
Google's Buzz has a huge potential privacy hole.
Scroll to the bottom of your Gmail page and click "turn off Buzz" unless you want people "following" you.
More in more detail here: Google Buzz Privacy Concerns
I've sent the url to several clients (especially the less tech savvy ones) so they can make the choice themselves.
UPDATE: There's far more involved in turning Buzz off. Read this.
The three steps to turn it off are here.
And check it out, Yahoo had "Buzz" a year before Google. It's even called "Buzz."