IMHO to have a firearm and pot on your person at the same time would be at your own risk.
Regardless of State Law, Federal Law makes pot an illegal substance. To purchase a firearm you would have to lie on the Form 4473 which would be a violation of Federal Law. A "user" of a prohibited substance is also a person prohibited from possessing a firearm.
http://www.justice.gov/usao/ut/psn/documents/guncard.pdf
I guess it's up to the individual to decide if they want to risk a Federal Charge or not.
The federal prohibition of marijuana existed in a legal gray area, so long as there was no legal source of marijuana within the national borders of the United States. The prohibition of intoxicants is one of the legal powers the U.S. constitution reserves to the states; The federal government simply lacks authority, even under the supremacy clause, for anything not specifically granted to it by the constitution, and doubly so for anything specifically denied to it by the constitution. This is why the 18th amendment was ratified in 1919, to override the 10th amendment to make federal prohibition of alcohol constitutional. The feds had exactly the same amount of supremacy over the states when it came to prohibiting an intoxicant in 1918 as it does today in 2012, and they needed to pass the 18th amendment to make prohibition legal.
The 21st amendment wholly repeals the 18th amendment, returning the law on prohibition to the state it was in in 1918. The 21st does clarify that the federal government has the authority to block interstate and international smuggling of intoxicants into any state where they are illegal, but this does not grant them the authority to prohibit any intoxicant within a state if the state does not wish to do so. Importing marijuana into Washington is only illegal at the moment if state law prohibits it; It sortof does, since marijuana must be approved by the liquor control board in order to be legal.
The U.S. constitution forbids Congress to pass any laws relating to things the federal government is not allowed to meddle with. An unconstitutional law is null & void from the moment of inception, not from the moment of successful legal challenge, according to the U.S. Supreme Court. A constitutional amendment that renders an existing law unconstitutional renders that law null & void from the moment the amendment takes effect.
A federal law that makes it illegal to possess a firearm while committing a crime becomes null & void (at least for that particular "crime") if the crime in question is one that is due to a law that is constitutionally null & void.
That being said, nobody would have ever invented police if nobody ever broke the law or violated an oath. We'd just be sworn not to ever break the law some time in childhood. Just because the law says something, doesn't mean people will obey it, even if they've sworn a sacred oath to obey it.
An interesting point of constitutional law though, is that NO ONE outranks an elected county sheriff when it comes to law enforcement within that specific county. If the county sheriff is on board with the legalization, then federal agents won't be able to enter the county to enforce unconstitutional laws without being subject to arrest, trial, conviction and imprisonment.
Then there's federal law. 18USC241 and 18USC242 make it a criminal act to use official authority under color of law to infringe upon the rights granted to any citizen by the laws of the United States or protected by the constitution. Violating these statutes is worth a year in prison for a simple oral discouragement; Ten years if a threat of a dangerous weapon is involved or two or more officials work together to violate rights, and is an actual for-real capital crime (life without parole or execution) if anybody dies as a result of the violation of rights. When was the last time you saw a federal SWAT team that was unarmed, or consisted of only one agent?
"Unlawful" under Federal law, or "Unlawful" under state law. It seems ambiguous.
What federal law? An unconstitutional law is null & void from inception, not from successful challenge. Prohibition of intoxicants is an authority reserved to the states under the 10th and 21st amendments. When Washington legalized marijuana, the federal prohibition of it within the state of Washington became unconstitutional.