I ride with a TiBook. It's got a few dings, but is working fine. Timbuk2 and Crumpler (plus others, I'm sure) do laptop inners that give you a bit of padding.

But the best thing to do is insure it. I can't imagine having a laptop that's not insured, since they get smashed and stolen so often. Just make sure your insurer will cover it when on your bike. That way, you effectively pay someone else to worry about it.