I had purchased two Wireless Mobile Mouse 6000 models late last year (2010) and both of them went defective early this year. They both had the overheating issue mentioned here (at the receiver). It caused Windows to not be able to recognize the devices,
also mentioned here.
I'd like to report that I called the 1-800-360-7561 number and Microsoft replaced BOTH of my devices for FREE. If you have this issue with your mouse or mice call the number and they will replace your device.
From what I've been told it seems that we are just unlucky purchasers. Apparently, there was a certain stock of these mice that has/had a defective receiver when manufactured. Microsoft told me that you can purchase the mice in confidence.
TL;DR Got mouse replaced for free by dialing 800 number, was victim of defective stock