This is a question that has plagued me over the last year, and this thread has been so helpful. I have not bought a laptop yet so it is difficult to pick the bag I want. Or maybe I should find the bag and then the laptop to go with it?
Here is the Tumi 3 Way (much less exciting than it sounds). You can carry it as a backpack, or with a shoulder strap, or with the handle. http://store.nordstrom.com/product/product.asp?styleid=2842794&category=2376779~2376805~2381640&PrevStyleID=2851815&NextStyleID=2842791
It seems practical, although my vanity will probably lead me to a Coach or Kate Spade. All my friends in law school say you need something with wheels, but that is a road I don't want to cross just yet.