I got a Denali Sport Deluxe Bag (made for portable DVD players) at Target for $20. My Sammy fits perfectly, with plenty of padding, and a velcro strap to keep it in place. It also has a separate area for my mouse and other things I need to take with me. Also, with a little finessing of this bag, I can get it to lay flat, which means I can get my Sammy through security at the airport without having to take it out.
I found some pics of this bag on Flickr with an Asus eee PC in it (see link below.) With a Sammy, there’s no room for the mouse and other accessories to sit where they’re shown in that one pic. But there’s plenty of room in the other compartment, which is where they’d need to be anyway if you’re using this bag at airport security.
It’s also small enough to easily fit the whole bag into a larger bag if need be.