Theoretically you should be able to collaborate over SharePoint Workspace without a SharePoint installation. SPW is largely the next version of MS Groove 2007, which was a peer-to-peer collaboration software that could sync with SP. SPW is more SP oriented, but it does keep that Groove peer-to-peer connectivity.

Unfortunately I think it's still too early to tell just how capable that peer-to-peer functionality will be. From what I've seen, the intention behind SPW was to allow users to sync libraries and lists from SP and then use them off-line. Peer-to-peer syncing is a way to share and maintain updates, but I have a feeling that the program is always going to want to dock with the SP mothership at some point.

If you limit to the calendar you should be fine. Just keep in mind that the way SPW syncs between workspaces is by having a complete copy of the workspace on each machine. If you're trying to maintain libraries in SPW, this could result in a lot of space and resource usage.

My $0.02, for what it's worth.

-SW