Federally, it's still illegal. However, there have been statements that they aren't prosecuting or pursuing legal action in states that have legalized it [0].
Edit: For the record, it would still block you from getting a US Government security clearance, and you can still run drug tests on employees and decline them for failing.
IOW: at the federal level illegal by law, ignored for recreational amounts by executive order (and thus possibly changing when a new executive branch is elected).
Federal enforcement is carried out by the DOJ, which is an agency of the Executive Branch. The DOJ serves at the pleasure of the President, so while Obama chooses not to enforce this law currently, he (or the next President) could change their minds at any moment.
In practice, Microsoft is betting on their lobbyists to ensure a stable regulatory climate.
[0]: http://www.denverpost.com/2013/08/29/federal-government-wont...
Edit: For the record, it would still block you from getting a US Government security clearance, and you can still run drug tests on employees and decline them for failing.