Read the OS X Guidebook2" by robg ;-) (linked in the box on the right of the screen). If that's not enough, then buy Mac OS X Unleashed.
I've been working on UNIX systems of different flavour for over 12 years in my daily work. I set up dozens of UNIX systems as webservers, bastion hosts, firewalls and gateways. I used Macintoshes since the mid 90-ies for professional and "home" work. I thought I knew my way around Mac OS X, been using it as my primary platform since 10.0.4 came along. Killed all my Linuxes, using my Mac for everything now.
BUT I did learn a lot, repeat A LOT! All the stuff that's not really in other UNIX systems such as HFS+, the way OS X does startup stuff, NetInfo etc. is explained at depth. But I guess you won't need to be "born" on UNIX to get a lot from that book.
The only negative point: it's about as heavy as my IceBook ;-)
-- skyko
[Editor's note: I have not read this book yet; I'm still working my way through David Pogue's also excellent Mac OS X - The Missing Manual. But Unleased is on my "I'd like to read!" list.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020221091202387