Disclaimer: I work at Microsoft (but this is all my opinion).
Dryad has been discussed publicly since around 2006 (iirc). Hadoop's earliest public release I can find is from 2007 (but I'm sure it was around before that).
I just want to put this into perspective. Microsoft isn't suddenly implementing MapReduce 6 years after Google released the paper. They realized as soon as the paper was released that it was a good idea. (Although Microsoft had some decent distributed systems already in place)
