Better make sure that what you use is up to electrical code. If your house burns down you may have issues with the insurance company. Home Depot sells grey wire specifically for this purpose and it meets code.
Low voltage wire (including speaker cable)should not have any code issues unless: 1. You are running it in a plenum (air return), improbable in residential construction. 2. You have a very draconian building code.
Chris, I point out this issue since some people don't know what their building codes are. I would hate to see someone get burned (pun not intended) by their insurance co because they didn't hire licensed people. I won't take the chance.
In the case of speaker wire, I would think that the low-voltage aspect of it is not an issue so much as the materials used to make the outer jacket.
If you use wiring that has jacketing that is not rated for in-wall use, you're essentially installing a really long fuse in your wall. Should a fire break out near your equipment (electrical for example), the fire may spread to other parts of the room/house quicker if you've got flammable speaker wire running all over the place. Try cutting off a small piece of you wire and setting fire to it outside and see what happens.
Here's a link to a PDF which discusses warious types of in-wall wiring.
I know the thread is a bit old, but does anyone have any experience with the 14ga in wall speaker wire at partsexpress.com that goes for $13.80 for 100'? Since I will probably only be in the current house for about another year or two, I'm not too concerned about high quality stuff.