Difference between 10.5 iCal and 10.6 iCal

Hey Folks I have a 10.5 Server, and have iCal working on it for the most part. Folks can access thru client iCal, and also thru the iCal on the iPhone. The one big problem seems to be the lack of ability to access these calendars from anything but Macs. We have someone on a PC(who can't be switched) and needs access to the calendar to see, edit, create, and delete events. Is there a way to do this on 10.5 that I'm not aware or and/or does 10.6 allow this capability ? Also.. does anyone have any experience with CardDAV on 10.5? How's it working out so far?