I followed every one of the steps you have in your sticky -- rebooted my WinXP PC as well as the Kindle 3 -- nothing helped. The information from "debug device detection" provided me with a message to copy & paste provided info somewhere, but there was no info to copy c&p! Strange...
What appeared to correct the problem was plugging the Kindle 3 into my laptop, also running Calibre -- that detected my device. After being reassured the problem was with my setup not my device, I re-plugged the Kindle into my main PC, fired up Calibre and -- inexplicably -- it was recognized. I tried it several times and all was/is well.
This is a very curious phenomenon I've never seen before. Somehow that "Kindle 2/3 Device Interface" plugin update was the cause -- can't explain it. But I disabled then re-enabled it which may or may not have helped fix my problem.
Thank you for Calibre -- and a speedy response on a Sunday evening