Depending on the type of trip I'm on and how much extra stuff I'm packing, I go with one of three bags:

For traveling light:
Waterfield Designs Vertigo w/laptop sleeve. When I can manage, I prefer traveling with it because its slim profile leaves plenty of legroom under the seat in front of me, and it can fit an IBM X40, a couple magazines, a book, cell phone, and computer gear with no problem. However, I'll often need to have more stuff with me.

For traveling not-so-light:
Mountainsmith Network. It has a built-in laptop sleeve, but I always use another one for extra protection. Slides over the handles of a rollaboard and has nice features like a water bottle holder, cell phone compartment, keyring, and an unzippered pocket in the back that's perfect for easy access to your boarding pass. However, I find the shoulder strap to be a little bit flimsy, and the bag may not be professional looking enough for some settings.

For giving my shoulders a rest:
Mountainsmith Explore. It has all of the same features as the Network (minus the boarding pass pocket) in a backpack format. Both the Mountainsmith bags have mondo padding, including a unique sleeve design that suspends your laptop just above the bottom of the bag. However, I find the shape of the backpack to be a bit too rectangular/boxy.

Still looking for the perfect bag...