I've been scrapping on a Mac for years.
I use Photoshop CS3 and Bridge for my layouts.
For managing my photos and my scrapbook elements I use iView Media Pro. This program allows me to sort by keywords/designer/date/event/color... What I like most about it is that I don't have to keep everything on my hard drive like you have to with iPhoto. I have WAY too many photos to keep on my computer. So I keep them on CD or DVD. iView maintains a thumbnail picture of the item in the catalog so I can find it. BTW, I also use the program to organize my finished scrapbook pages and even my music collection.
Don't forget about Font Book to find just the right font for your layout.