I honestly do not know, as I didn't work with iCal Server in the the pre-release beta stage. I am hoping that iCal Server, like the open source Darwin Calendar Server, will work with iCal 2 and virtually any release of Mac OS X 10.4.s, but until it ships and explicit documentation is available for it, I won't know. Both Mac OS X 10.5 and Mac OS X 10.5 Server ship in eight days, so it won't be long before we know.
I haven't created a multiuser test environment for the version of iCal which ships with Mac OS X 10.5, so I do not know if 'sharing' in any form is implemented with the client version of iCal at all. I suspect that either iCal Server or the Darwin Calendar Server will be required to share information. For companies, this is a simple, reasonably affordable no-brainer. For individuals, Mac OS X 10.5 Server 10 Client—half the cost of the Unlimited Client package—is still a significant investment, particularly when you consider the cost of the additional hardware (short of an Xserve, with which OS X 10.5 Server is bundled) to host it.
I expect that you will see subscription based CalDAV-compatible commercial servers pop up soon, which can host your calendaring data and will require only the use of the client version of iCal or a CalDAV compliant Windows alternative such as Mozilla's
SunBird cross-platform calendaring application. You may see Google, Yahoo and
Zimbra incorporating CalDAV technology into their offerings, as well as
Plaxo.