For anyone looking to cut their cash outlay and maintain multi-Mac data sync, there are options for calendaring (BusySync, Google's CalDAV support) and some for address books (address-o-sync, or the free Google & Yahoo sync in Leopard's Address Book), and some products that handle both calendars and contacts (SpanningSync, Plaxo) -- but if you're in the mood for a free all-in-one approach, you may want to check out a promising beta service from a team of European developers: fruux.
Starting with address book sync in earlier versions, fruux has now progressed to include calendar, task and bookmark sync via Apple's built-in apps (Address Book, iCal & Safari) and a cloud service that stores the synchronized data. The system pref pane interface is no-frills and the product is definitely still a work in progress (there are prominent warnings to back up your data beforehand and frequently thereafter), so it may not suit your needs just yet -- but it's definitely one to watch. Future roadmap plans include an iPhone client and a web interface to your PIM data.
Thanks Dominik, Hagen & everyone who sent this in.