I got an IPEVO Interactive Whiteboard System ($149 retail price) to demo and did a first little experiment. The product consists of a video sensor and interactive pen. You have to have a projector as well. Here's a super rough demo I made for my team at Balsamiq to show how it would work using Balsamiq Mockups.
While the demo is pretty rough given the 15 minutes I gave myself to set up and record with my phone, it's pretty interesting to consider the possibilities for whiteboarding with wireframing software, if you're so inclined. I used a regular painted wall, but I'm going to give it a try on a real whiteboard.