NO problem whatsoever Ken. I used a D40 with my RC8 for over a year. Below are a few of the images I did with that setup. All taken from 2008 to 2009. No vignetting present in those images. Some vignetting in the subs but using flats got rid of those problems.
http://paulhaese.net/trifidcloseup.html
http://paulhaese.net/NGC5128closeup.html
http://paulhaese.net/NGC4945.html