Not completely - but you're far better off than with a PC.
However, while OS X is naturally more secure than Windows XP, it's not going to be perfect and the typical Mac user doesn't have that sense of paranoia that any competent PC user develops. I'd guess they're probably more vulnerable when someone does figure out how to write a nasty.
I've also switched to a iMac (G5, 2Gig Ram, Wireless everything) and it's fantastic. A couple of early niggles with Bluetooth and especially WEP, which has been fixed by a recent sofware update, but apart from that...
I still run an XP/Pro box for miscellaneous stuff that isn't available on the Mac, but it's hardly used
