Instapundit has a good link to first hand reports back from Ukraine from election monitor and former Congressman Bob Schaffer. If his reports are correct, Russian Special Forces have flown into the country, dressed as Ukrainian forces, and are "defending" the presidential palace.
While Russia has an impressive history of involving itself in its neighbor's affairs, this is not a good sign. The strong US response that "the United States stands with the Ukrainian people in this difficult time" will hopefully be followed up with quiet diplomacy stressing to Russia and to Ukraine, that fair elections are to be expected in the former Soviet Union.
May justice prevail, and democracy's voice be heard.