I agree with those answering "this isn't an ethics question". I'm no expert, but one definition of ethics reads "that branch of philosophy dealing with values relating to human conduct, with respect to the rightness and wrongness of certain actions and to the goodness and badness of the motives and ends of such actions." The law has defined the "good vs. bad" aspect of the ends and the motives are no different than any other project.
The question posed seems to be clearly a legal question... unless you're morally against the medical use of marijuana. In that case, it'd be the same dilemma you'd have with providing engineering services for the construction of an abortion clinic, KFC chicken ranch, or Wal-Mart. That is, are you willing to walk the walk having talked the talk? As much as I hate Apple in general and Steve Jobs in specific, I admire their conviction of censoring what they believe is pornography on the iPad - I just hope it hurts them in the bottom line and people stop buying their beautiful, but overly closed and restrictive, products.