Just to let y'all know, there is no constitutional crisis because there are only eight justices sitting on the US Supreme Court.
There is nothing magical about the number 9.
Here is Article III, Section 1 of the US Constitution:
The judicial power of the United States, shall be vested in one Supreme Court, and in such inferior courts as the Congress may from time to time ordain and establish. The judges, both of the supreme and inferior courts, shall hold their offices during good behaviour, and shall, at stated times, receive for their services, a compensation, which shall not be diminished during their continuance in office.
See? There is nothing in the Constitution that says there have to be a certain number of justices on the Supreme Court. And, in fact, there is nothing in the Constitution about the qualifications of the justices, either.
There were only six justices when the Court first sat (and only four of them sat). Over the years Congress increased the number of justices when new federal circuit courts were created. The number peaked at ten in the 1850's. That number was winnowed down to seven when Congress passed a law stating that the next three seats to open through death or retirement would not be filled. The number was then increased back to nine when two new circuits were created.
Franklin Roosevelt caused a stir when he proposed appointing a new justice for each sitting judge who was over the age of 70. That proposal never got off the ground as both Democrats and Republicans in Congress opposed it.
Whether or not the Republican-controlled Senate should hold hearings on President Obama's nominee to fill the seat held by the late Antonio Scalia, is merely a political question, not a constitutional crisis. There is nothing to prevent the remaining eight justices from hearing cases and issuing rulings. A tie just means that the lower court ruling stands.