IMO, it'll take a while for Macs to get a considerable larger total market share. Right now, I believe Macs have 5-6% market share, while Windows has something like 90%. A lot of people will will get PCs because "Well, most of the world is a WIndows world so I might as well fit in." With Leopard & Boot Camp, it should help bring some more people over.
As for security, having such a small market share is a good thing in some respects b/c it lets us Mac users have security by obscurity. So basically, why would you want to hack a computer platform w/ only 5-6% market share when there's a platform w/ 90%? If/when Macs get more market share, don't be surprised to see more malware, people trying to hack, et al, Macs.