I still have no opinion on baby stuff, but if you're looking at messenger bags, I'd suggest looking at Tom Bihn. For one thing, unlike Timbuk2, none of their bags are made in China.

I got one of their Checkpoint Flyer bags, and it's great. It works as advertised at airport checkpoints. And it even got a positive comment from a TSA employee. It's probably not the right model for a diaper bag replacement, but I'm sure one of their other models would be.