My original setup that I used for a couple years was a regular backpack with a grocery pannier. It worked great, because I could just pop the backpack out of the pannier when I got to work. Plus you can use the pannier for other things like grocery shopping or whatever.

The one I have is the Jaand grocery pannier. It's a bit on the expensive side, but it's very solid. There are plenty of other brands that you can get a little cheaper.

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