I highly recommend a messenger bag from Jandd Mountaineering. This is the model I have used almost every day for the past 10 years:

http://www.jandd.com/detail.asp?PRODUCT_ID=FGMB

I believe their construction is much sturdier than the Timbuk2 products. I have a good friend who has worn out 5 Timbuk2 bags while my Jandd bag is still going strong. The stitch count is much higher at the seams, etc.

My bag has a lot of small compartments, including a great PDA-phone-sized pouch for easy access. The main compartment is cavernous and has an expansion zipper to allow for even greater capacity. I find this option useful on trips because I pack my stuff with the zipper closed, and I can unzip it to allow for new acquisitions on the return home. I made a 2 week trip to Europe with only this bag.

It has a very thick, padded shoulder strap and a concealed belt strap if you need it. It also looks like they are making a special laptop insert, which was not available when I bought mine. I carry my laptop to work every day in this bag without any special padding, but a little extra might be nice.