I reset my accidentally the other week too. For one reason or another I had the MFD display on 'long term' for all the average readings (speed, MPG, journey time etc) and forgot about it. After refuelling, I went in to the MFD and reset the figures, thinking it was the 'since start' figures, with a prolonged press of the "OK" button - and they were gone! Damn! My since start weren't far of the long term so wasn't immediately obvious..
Oh well. I track the MPGs on Fuelly as well as a cracking little app called Road Trip (I've done this for every car I've owned, including my XJ6) so not a complete failure..annoying more than anything!