We use Sharepoint 2007 for our intranet portal using Windows integrated authentication with no (or few) problems for Macintosh users. I just logged on to ours from my MacBook and was able to access it with no problems. Can you describe what the problems are in more detail? Not able to log on (should they be specifying a domain -- we have to)? Controls broken? Are they standard or custom controls? If custom controls, maybe the javascript isn't compatible with Safari. Speaking of that does it matter which browser they use? i.e., do you see the same problems in Safari and Firefox?
EDIT: Just found this tidbit on our intranet site that may be helpful to avoid all the password prompts, though it requires Firefox. I'll poke around some more to see if I can find a solution for Safari:
Using Firefox on a non-Windows workstation (e.g. Linux):
In Firefox, type about:config in the URL address bar and press Enter
Find network.negotiate-auth.trusted-uris and double-click it
In the dialog box that comes up enter:
your-domain.goes.here.com
Click OK (Firefox should now use your Domain credentials to access these sites)