I've been exploring this as well. I carry a thinkpad x31 (mine rather than corporate) at about 3.5 lbs + t-shirt + shorts.

So far:

* Aerostich Dispatch bag left over from the motorcycle experiment - Heavy, wandered on the back, never as tight or as stable as it was on the motorcycle.
* Timbuk2 Pee Wee - Wandered on the back, really didn't try the chest stabilizer strap since it didn't seem to go around me.
* Some backpack from REI with a water bladder.

I tried the last today and it certainly was a lot more stable though i had pulled the water bladder. A little more room too.

What is the advantage of panniers over backpacks/messenger bags -- just more flexible and keeping your back cooler?