I Just Bought a 2000 450S And noticed a lot of hairline cracks in the rear Differential so I went back to the dealer and looked at some showroom models and the had them too. The Service Department said its Just the way it was Cast and Shouldnt be problem. Take a look at yours closely
and give me some feed back ?????? RIDERED (I Think)

 

I looked at our okd Honda & there are what appear to be cracks in the rear diff. Been there 6 years, though. I bet if you took a file & smoothed them off, they don't go below the surface.

 

It's nothing to worry about. Your dealer is telling you the truth, I owned a 450S and it to had the same hairlike cracks in the rear differential but they cause no problem.