I mounted the Garneau bag on my road bike last night. Not really happy with it, but it was still a fair deal for $13 on sale at Nashbar. It has one of those plastic QR brackets that clamps semi-permanently to the handlebars, which I don't like. I want something with a bracket that goes easily on and off as needed, so that my bars are free for my computer and lights the rest of the time. It also sags down a bit because it is all hanging off the bracket at the back and the internal supporting wall is a flexy piece of plastic.

Here's what I was describing earlier, which hooks over the bars & stem in the same manner as what Rollinalong posted earlier.

Rhode gear used to make a bag mount that hooked over the stem in the same manner, but was supported by a thick piece of wood inside the bag and a bungee that went under the head tube. It was very stable. Couldn't find a picture of one, however.