I have tried using SB28 on my Contax 645 before. Actually, Nikon USA had replied my letter saying that it would be fine to use SB28 on Contax body without covering any contact point.
Frankly, I was surprised when I got their letter because I guessed they should seldom answer this kind of "using off-brand items on their products" questions and by formal letter as well.
_________________
Please visit my photo gallery and your comments are always welcome!
I will add photos to it from time to time. Thank you!
http://www.flickr.com/people/yualbert