Former German Chancellor Schröder has finally brought himself to saying that his friend Putin is violating international law in Crimea. Interestingly he now thinks that he himself did that same thing with Serbia, which is why he doesn't want to condemn Putin. (It's interesting because a) the comparison was brought up in this thread too and b) that was a very controversial thing here back in the 90s, with the first case of German military fighting since WW2. Of course at the time he had another opinion and (I'm being mean here) no Gazprom job yet.)
His assessment that the EU is partially responsible for this, with the EU association agreement and the Russian customs union being mutually exclusive rings true though, that seems like a miscalculation now, if the particular Ukrainian situation is considered.
The USA has warned Russia that they must not annex Crimea or that will close the door to diplomacy. Exactly what that means I do not know. I don't think Obama knows either.
Since they are barely talking now, probably economic sanctions I guess. Europe continues to talk with Putin, but it looks like he is going to push the referendum through. The West will consider it illegal, but for practical purposes this will not matter much. What happens then will remain to be seen. Economic sanctions will be hard on Russia, but hard on Europe too. Not only because of oil and gas, but because account freezing and wider visa restrictions will negatively affect Russian investments and tourism, which is quite important in some regions.
Since even the US need Russia (to resolve the stuff with Syria for example), it is at worst possible that - if this remains limited to Crimea and bloodless - this could be forgotten like Georgia all over again in a few years, just with a longer phase of sanctions and a new low in international relationships.
The Tatars thing - that's bad, but I guess that was to be expected.
I have also read that there is a (so far tiny) contingent of Serbian Pan-slavic militia in Crimea, to support the Russians. (Serbia's official position is against Crimean secession, because of their stance on Kosovo. They plan on punishing these guys when they come back.)
That goes to show though, there is a lot of potential for conflict there, even if it does not come to anything with the military.
There are also reports about Russians travelling to Eastern Ukraine to support pro-Russian protests there.
All in all lots of worrying signs that the situation is not exactly stabilizing. With so many different armed groups involved now, a lot could go wrong.
If the Russians invaded western Ukraine at this point, NATO would declare war. The only way Russia could get away with that is if Ukraine shot first.
I highly doubt that, as bad as that would be for Ukraine. I don't know what would happen, but military intervention is not on the table. Even the Republicans have made that clear. It just is too risky, you can't start a war with a nuclear power, hoping that they only use conventional weapons if you do the same. You think the Russians will just sit there and hope the rockets and bombers flying at them won't have nukes in them, so they will respond accordingly to keep it a fair fight?
That's madness, no way that's gonna happen.