I've got one of these
Targusomg|omgCN600omg–omgTargus 15.4 - 16 inch / 39.1 - 40.6cm Laptop Backpack
I spent ages looking online but Oz is a backwater for online shopping - I ended up taking the laptop with me shopping to an actual retail outlet. This has a removable padded 'sleeve' for the actual machine and this is held in place but a velcro tied elastic strip. It has 2 small side zip pockets and 2 other compartments on the back. I can fit the laptop and power supply and a bit more stuff in the main compartment. I stick the mouse and headphones into the smaller back section and have a relatively sizeable space that is about the size of an A4/Letter size notepad (but about an inch thick).
It works pretty good, but I kinda wish I'd bought something with a bit of extra space in it - and I also wish it had a hip strap - my alienware is a heavy beast - walking any distance wearing this - it can get a bit uncomfortable.
I'm spending 6 weeks in the UK next year - this is where I'll really see how this goes as a commuter bag.