Article VI of the Constitution says:
This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land; and the Judges in every State shall be bound thereby, any Thing in the Constitution or Laws of any state to the Contrary notwithstanding.
This is known as the "Supremacy Clause."
What this means, in a nutshell, is that if the federal government passes a Constitutionally proper criminal statute, the feds can arrest you no matter what state law says.
That means if your job requires you to possess pot, you can be busted under federal possession laws. If your job requires you to distribute pot, you can be busted under federal distribution laws. And even if you work as an accountant, you may be subject to federal prison time under federal RICO laws.
Now, as a practical matter, the feds in general (but not always) have been taking a more hands-off approach to busting dispensaries in legal states. But they're choosing not to bust them; they're not powerless to bust them. And that could change. At least two of the Republican candidates for president (Christie and Rubio) have said that they think the feds should enforce anti-drug laws even in states that have legalized drugs. If one of them were to get elected, and if he had enough support in Congress, that could change very quickly.