I had my D80 converted to B&W by Lifepixel earlier this year. They replace the filter in front of the sensor. I had them tune the focus for my 18-200 and I just leave that on the camera. I am very happy with the results.
When Lifepixel tunes the auto focus they are tuning it to be optimized for infra red which is different than visible light. My D1X IR has had the AF fine tuned and I find it is right on with all the AF lenses I've tried.
The D 90 will do B&W,(with filter effects thrown in) all by its lonesome. You can do all en camera!
Go to "Set PICTURE CONTROL" (camera icon) on the SHOOTING MENU, scroll down to MC (MONOCHROME), then scroll down to the filter effects you wish to use, if any. RED, Yellow, Orange, Green etc, then the type of print you want...Straight B&W, Sepia(brown tone), or Cyanotype(blue and white). I remember countless hours, agitating prints in Sepia toner, trying to get what the Sarge said was the "right" tone!
This is MUCH better! O my, yes!
It might even save you the $$ of the conversion. Then, buy a new lens
Given the Leica Monochrom experience, there is benefit to having an only B&W camera. Given the buzz elsewhere (and I may have missed it on Nikonians), there does seem to be some interest in a commercial conversion of existing traditional "color" DSLRs to B&W. It entails at least in removing the Bayer filter but I'm not sure about the rest. I've inquired, but don't have an answer yet. By removing the Bayer filter you increase the resolution possibilities of the sensor. You also eliminate any RGB histogram balancing due to the imperfect response of the various Bayer elements. However, without knowing more, I think the latter would be part of the in-camera processing. So I could speculate to no real benefit, but I'm researching this as I have a body that I might consider doing this with.
When I look at the Leica Monochrom files, there are distinct tonal advantages of having a native B&W sensor in the camera over having to do the conversion in post. I have my own methods for post to B&W and I've experimented quite a bit with Silver Efx Pro, enough to know that I will be adding that software as soon as I'm done migrating computers and my Lightroom libraries.
Roger It's still, ISO, aperture and shutter-speed, right?