"Halt!", says my Macbook. "Who are you, and where are you going?!"
Anyone who's ever been on a large network would know the intricacies of granular file permissions. Access Control List (ACL), some people pronounce it like "uncle", have been around long before but during regular conversation you'll most probably hear this from people administering a dominantly Microsoft Windows network.
On Windows XP you get this granular feature if you turn off the easy file permission management which opens up other management tabs such as Audit trail when you start to share files and directories. Mac OS X has a similar neat feature better than Windows in its own version of the Control Panel called Shares under System Preferences.
What was missing was granular ACL ... until now. Have a gander at Sandbox for your specific sharing needs. BTW, it now works for Leopard, too! :-)
Postscript following morning at 9am:
Argh, just to say where no one has gone before is not the case, someone else made a few comments about the new file sharing properties on Mac OS X here.