Thanks for the tips...

I ended up picking up one of these:

http://arcteryx.com/product.aspx?lan...s&model=Kea-45

I wanted 45L as I expect to be taking the PJs and amenity kits with me from some flights - I'll probably fill up just about half when leaving.

For some segments I really don't mind checking the bag in if need be - I have a fold-up bag that I can use to carry my iPad and camera in if I need to check the backpack in.

Other times I don't want to have to wait for checked luggage. For instance, I arrive NRT on NH1009 and every minute will count in making it to the last Narita Express train.