While it's certainly possible that this a mule being tested with the fifty-state legal diesel that's coming to the US market in 2009, it's unlikely. Honda R&D is based out of the company's US base near Columbus, OH and if they were testing on the street it would probably be in a US spec body with no special badging.
Prototype Hondas are rarely seen in the Detroit area. This one probably belongs to one of the domestic car makers who regularly procure foreign models and bring them over for benchmarking purposes. It's not at all unusual to find vehicles like this one being driven around with "M" plates.