The Ninth Circuit just held (2-1) that the federal ADA excludes from coverage current illegal drug users and that "illegal" includes marijuana use that would be lawful in California, but is unlawful under federal law:
We hold that doctor-recommended marijuana use permitted by state law, but prohibited by federal law, is an illegal use of drugs for purposes of the ADA, and that the plaintiffs' federally proscribed medical marijuana use therefore brings them within the ADA's illegal drug exclusion. This conclusion is not altered by recent congressional actions allowing the implementation of the District of Columbia's local medical marijuana initiative.
The case is James v. Costa Mesa and the opinion is here.