I think you're in for a fun ride. I assume you're a Windows user (XP?), and OS X isn't all that different from Windows in terms of most usage. Since the PB doesn't come with a two-button mouse area, remember that holding down 'control' while clicking is the equivalent of a right-mouse click (and, of course, you can always plug in a multi-button mouse - and also, realize that 95% of the Mac pro community thinks the one-button mouse thing is dumb).
I highly recommend picking up "Mac OS X Panther Killer Tips" by Scott Kelby - it contains paragraph-long tips that you can assimilate at your own pace, and you can just flip back and forth and learn things randomly. It's a nice place to pick up some of the cool things.
Also, if you can't figure out how to do something, post here. 95% of the time, someone will answer quickly. If not, bump the post later, and someone will answer then. If all else fails, PM me and I'll answer.
I think one of the biggest differences is that closing a window in OS X doesn't always close the app - it just closes that one window. However, OS X multi-tasks very well, so this isn't a big deal.
And once you open a bunch of windows and apps, enjoy pressing F9, F10, and F11 (the Exposé keys). Great fun.
I think you should probably play for a bit, then post back with any issues.