While it's not a backpack per-se, I use this
rolling pack from Targus.
It holds my 9750, the Logitech DiNovo Edge
I use with it, along with all the accessories I need for the system (i.e. power, headphones, travel mouse, mousepad, cables, etc.)
As an added bonus, I was able to pack away all the gear for my PSP in the detachable mesh pouch, and the large rear section (designed for a change of clothes) is roomy enough to hold my Xbox 360 with all of it's gear and 2 controllers should I be going to a SERIOUS LAN party!
I know it may sound like overkill (that's what I thought @ 1st too), but I absolutely LOVE this case. I got mine @ Best Buy for about $99 on sale.