Two and a half years later I too come across this site; wish I'd found it earlier.
My understanding of the derivation of the name is similar to Chris', but not exactly the same. Unfortunately I can't quote the source, but it is one of the standard works on the history of British surnames.
I'm sure that Chris is correct in saying that the name originates from a place near Blackburn. Most of the earliest recorded references seem to come from there, and Dewhurst still exists today as a farm on the north bank of the Ribble, 9km north of Blackburn and two km east of Ribchester.
However, I've always thought that the name derived from the Welsh personal name "Dewi", meaning David. The name would then mean "Dewi's clearing". My sentimental and simplistic mind has then always linked this Celtic connection to the several Evan or Ewan Dewhursts who appear in the early parish records of the area.
Out of interest, am I placing this response in the correct chain or, after two and a half years, should I be starting a new one.