From the 30 Boxes blog: "Mike has found a great piece of software that lets you subscribe to your 30 Boxes iCal feed (in My Settings > Syndication) in Microsoft Outlook!" It's called Remote Calendars and it indeed does what 30 Boxes says it does. Give it a SwampDrain factor of +3. It implements what Microsoft demoed last year at Gnomedex, only in open source. Hooray!
Unfortunately, it still requires you to publish your calendar to the Web on a page, and worse yet, without any access control available for that page.