[SmokingJoe]

Depends on how you use them. If you navigate with IE (will the latest patches) to unknown websites you will eventually get zero-dayed, no doubt about it. With Safari, that is not as likely, but still possible.

How so?
Given that IE 9 is more secure than Safari.
The myth of the “safe” OSX has long been destroyed:

Mac Virus Infects Over Half A Million Computer
http://thebottomline.as.ucsb.edu/2012/04/mac-virus-infects-over-half-a-million-computers

Widespread Virus Proves Macs Are No Longer Safe From Hackers

http://bits.blogs.nytimes.com/2012/04/06/widespread-computer-virus-indicates-mac-users-no-longer-safe/

[palmer]

http://www.zdnet.com/blog/security/pwn2own-2012-ie-9-hacked-with-two-0day-vulnerabilities/10621