It is probably not very well-known for many, and probably ignored by most, but it seems that Mac OS X and specifically Safari leaves much to be desired when talking about security.
During the Pwn2Own contest, Safari was the first browser to fall, in the order of seconds, when put under attack by Charlie Miller. This has been reported in several places, like Pwn2Own 2009: Safari/MacBook falls in seconds, or Miller: Safari on Mac First to Fall During PWN2OWN Contest, or Miller Cracks Safari Within Seconds, Wins PWN2OWN Contest. For the second year in a row, Safari/MacBook has been the browser to fall under attack the first.
So, if you are a user of Mac OS X, be very careful when using Safari. These attacks so far require you to click on links specifically crafted to cause harm to your computer, which might allow the attacker to gain total control of your machine. Hence, the importance of never running with an account that has administrative privileges.