Again, that ruling was a few years after the civil war and pertained to Texas. While it does have some loose affiliation with the issue at hand, it doesn't address the issue at hand. Namely, the legality of secession. ....and sorry, I am one that does NOT think the court system is infallible. Kelo vs. New London, Ct. should prove that.
As for secession, it would have to be specifically written into the Constitution if secession were illegal as that is the document that places limitations upon the federal government. The 10th Amendment that states all rights reserved to the states and people respectively give the states, comprised of their citizens, the right to form their own government should they find that their current government is working against their best interests.