Since my telescope is down awaiting a new mount I don't have anything new to work on. So I decided to go back and reprocess some images I took before I had Photoshop CS. This M51, which was my first DSO image, was taken with a C14 and an ST2000XCM at f5.5, eight 15 minute exposures.
Thanks for the comments Ivan and Tyler. The only thing more I know to do with this data is to look at each individual exposure and see if I can isolate the source of the star elongation - maybe it's just one exposure, which I could shift or not use it in the stack.