Tom Bihn bags
here
I use the Ego messenger bag with the Brain Cell sleeve for my 15" MacBook Pro. It's beautiful. With the reflective stripe and clip-on CatEye taillight are nice safety options. The "SuperEgo" bag would accommodate your 17" easily. The only downside is the price, IMO. It's a great option if you can swing it. OTOH, you can't afford to NOT have your laptop very well protected if you're carrying it on a bike. (I have no connection to Tom Bihn, FWIW.)