Today's referendum in Crimea, conducted with Russian troops patrolling the streets, offers voters a choice between two options neither of which is acceptable to the government of Ukraine. In fact, pro-Ukraine voters appear to be boycotting the election, thus providing even greater assurance that the vote will show a strong preference for Crimea's reunification with Russia. The result, like the vote itself, will be rejected by the Ukrainian government and the governments of the United States and the European Union. This sets up the possibility that Russia will annex Crimea and the status of Crimea will fall into a form of legal limbo that persists for decades.
In 2008, Kosovo declared its independence from Serbia. Most UN member states (including the United States) recognized Kosovo's independence; Serbia, Russia, and China did not. Later in 2008, following the Russo-Georgian War, Russia recognized Abkhazia and South Ossetia as independent states; the United States (and most UN member states) did not. Of course, each of these cases differs from the Crimean case in that new states were created. The most likely outcome of events in Crimea is that Crimea will be annexed by Russia. This means that adverse impacts--sanctions, to be more specific--will be directed at Russia rather than Crimea itself. (Kosovo, Abkhazia, and South Ossetia remain outside the United Nations--and likely will remain outside the UN as long as the U.S. and Russia have vetoes in the Security Council. This stamp of illegitimacy will not apply to Crimea if it is annexed.)
Meanwhile, for those interested in thinking more about the mutability of borders, Wikipedia's list of post-World War I border changes is worth examining. There are many possible takeaways from the list--the role war plays in boundary changes, the potential for international judicial settlement to effect peaceful change in cases of border disputes, or the ongoing influence of the national self-determination norm as a maker of boundaries--but perhaps the most obvious and important point is simply that boundary changes are not terribly unusual in the modern international system.
Finally, NPR's Greg Myre asks, "What Are the Rules for Changing a Country's Borders?" Many of the commenters seem convinced that might makes right.