Hmm.... thanks for the responses so far! Yogajess, I was just looking at the Seagull bags, and they seem very interesting indeed.

To complicate matters, I came across this thread in SSFG:
http://www.bikeforums.net/showthread.php?t=299121

I had written off backpacks, but now I'm wondering if an Ortlieb Flight might be worth considering. It looks like a great bag, and the mesh back for warm-weather cycling sounds promising.

d2create - that sounds like a scary crash. Hope the Rivendell (which I had printed out and posted on the cubicle at my last job... new place doesn't have cubicles ) was unscathed too. A friend of mine had an Ortlieb roll-top messenger backpack save his back in a similar situation, but his bike was toast.