Why not keep track of your mileage in a log? Then you don't have to worry about your computers.
The computers I've had lose all their miles when the batteries die. They blank out when riding in sub-freezing temps, or when they get soaked with rain. They tend to reset themselves after a certain distance (I've never had one record accurately for a full 1200K randonnee). And they can leap off your handlebars and disappear in the ditch. Also, when you ride several different bicycles and have a computer on each, you've got to add it all up anyway for total mileage.
So, I use an Excel spreadsheet to record my miles, and any information I deem particularly important. I also use two online logs:
Bikejournal, which is used by thousands of cyclist around the world: http://www.bikejournal.com/
Big Dogs, for those of us who log a lot of miles, who are into ultra-cycling: http://www.big-dogs.org/scripts/bdhome.asp