So I recently noticed problems with my 35mm back focussing, and as my D90 doesn't have the fancy on-camera calibration options like later models, had the lens sent away for repair.
All I had to do was print off a focus test chart from here (unfortunately the original has disappeared off the face of the web, but archive to the rescue!), take a couple test shots at various apertures, and mail it off to get fixed. One month later, the lens is back.
Below is a comparison at f1.8 before (right) and after (left) the repair... definitely worth having done!
Big thanks to the awesome people at Snapshot!