That is the claim put forward, with gusto, by Jonathan Eyal of the Royal United Services Institute in this Guardian article. Eyal correctly notes the importance of the principle of distinction, and more specifically the obligation of combatants to distinguish themselves from civilians. And if it were true that Russian forces in the Crimea were ditching their uniforms for civilian garb, then we would indeed have cause to object. But my understanding is that this is not at all what is happening. Instead, as the Guardian account suggests, the Russians remain in uniform but have ditched or obscured their identifying badges linking them to Russia (and fooling no one in the process, it is worth noting). This is not obviously the same thing; if they still remain manifestly distinct from the civilian population, there is no violation of the principle of distinction. [Note: A reader points out that my comment about soldiers in civilian garb might be read as asserting that simply wearing civilian garb is itself clearly a criminal act. My bad, I had not meant to take a strong position on that point. In that scenario, we certainly have conduct inconsistent with distinction, and possibly have grounds to deny combat immunity (in the event of prosecution) or POW status (in the event of capture). Whether we also have a war crime in and of itself is a complicated question, at least if the fact pattern does not involve the exploitation of the civilian garb to enable the soldier to carry out an attack (i.e., perfidy).]
Of course, this issue only arises in the first place of the law of armed conflict applies. Does it? That depends on whether you think the government in Kiev is legitimate. If so, then there is a state of partial occupation by the Russians that would seem to implicate the Common Article 2 trigger under the Geneva Conventions of 1949. If instead you take the Russian view, and categorize the deployment in the Crimea as being at the invitation of the legitimate authorities...well, if you buy that premise, then the situation certainly has the potential to become a non-international armed conflict but has not broken out into violence yet and hence isn't one at this point in time.