Here's an idea for y'all...
I just finished making this so I thought I'd share it. It's an LED closet light that is triggered by a magnetic thingy you put on the door. I found this in a box of junk in my storage room and I think I paid $2 for it a little while ago at Princess Auto (Canada).
Inside the light there is a circuit board (which contains the LEDs and "magnetic" switch) that the batteries connect to. I cut the circuit board off of the battery leads, put the battery leads in series with my USB cable and momentary NO switch. Easy peasy. Looks good too.
After making holes for the switch and USB cable and a little krazy glue to hold the cable securely, I put it together and tested it with my SD980IS/IXUS200 and it works like a charm.
What I like the most about it is that this thing will never have to be opened again unless the switch wears out, since the batteries (3x LR44/AG13) are behind a slide off cover.
I could have left one of the LEDs in the circuit, but I wanted nothing to be able to suck the batteries dry while it is in the camera bag if the switch gets pressed. Since there isn't a complete circuit when it's not connected to anything, the batteries should be safe when not in use.