Hi, This sounds great, as do a few other availability modules out there such as:

All of the above however seem to have been abandoned or at least not kept maintained.

If it were possible to grab the best bits of all these projects and turn them into a generic resource availability module with plugins for users as resources, nodes as resources, UI, booking resources, buying resources with Ubercart/e-commerce, this would be one of the best things to happen to drupal in my opinion.

Use of the existing Date and Calendar modules would be a sound decision for interoperability.

I'm not sure where this functionality may exist already but I would love the ability to use an external calendar such as Google Calendar or other system that allows publishing of free/busy data via iCal or CalDav.