Ah throwing MAC's in to the bundle doesn't help.
earlier versions of outlook supported a rudementry calender sharing which was officially horrible.
Its one of the features that exchange does pretty well.
I'm guessing that your budget woudn't streach to SBS + enough calls for 20 people ( would be between a grand and £1500 for software , and you'd need somethign to run it on.
all however isn't lost , have you thought about a shared calender application such as sunbird ?
http://www.mozilla.org/projects/calendar/sunbird.html
if you have mod_dav installed on your linux server you can use it to publish and share your Mozilla Calender ( sunbird )
I'm sure this woudl work for the mac guys too !
www.twilight-systems.com/flacco/mozcal/