This just happened with my Trek Incite11i computer. The cadence worked but the MPH did not. I took it to my LBS and they determined it was the wiring. I got my computer in 2008 and there was a 2-year warranty so my LBS replaced the computer with a similar one since they didn't have a spare wiring harness in stock.
Edited to say...since you've only gone 600 miles, I'm sure there's some kind of warranty involved. Did you get it at your LBS?