Wireless webcam streaming
Combining the Sheevaplug with a USB WLAN card and a USB webcam results in a setup that can allow live video streaming over the internet, or perhaps as a surveillance / security camera.
I used the Creative Live! Cam Chat IM (model VF0530) which shows up in lsusb as Bus 001 Device 007: ID 041e:406b Creative Technology, Ltd. This device works nicely as a Video4Linux2 (v4l2) device, and is automatically configured when plugged in - no drivers needed.
Streaming live video over HTTP and setting up motion detection
For serving the webcam video stream over HTTP, I installed Motion (wiki). Motion has a built-in HTTP streaming service, and a ton of options. Although I didn't go much further than just seeing it run, there should be options for configuring motion detection for automatic archival of video snapshots, and so on. With a USB WLAN card, you can install this setup anywhere you need as long as you have WLAN coverage and a power outlet.
(The screenshots are lower quality than the real thing, I forgot to save the originals and so I had to grab the re-encoded images from my facebook photo account)