One of the coolest things shown at MIX08 was Deep Zoom. Deep Zoom is a technology that came from a technology originally called Seadragon. The demo that was shown at MIX08 was by Hard Rock Cafe. They have a huge collection of memorabilia so they built a Silverlight application made of a montage of 512 images (various pixel dimensions) and you can zoom in and out on them. Very neat.
Try it out at the HardRock Memorabilia site.
Another example built by one of our guys, Joe Stegman, is a version of the Silverlight v2 poster. Easy to move in and out and pan around the image.
To learn more about Deep Zoom I'd recommend a couple of blog posts. First I appreciated Scott Hanselman's post. The second, and where you can grab the tooling is the Expression Team Blog. User guide is there too.