Given that we are in the cloth forum, given the thread name is "Maya Cloth?" and given the actual question was "So how do I create a closed curve for cloth? " I assume we're refering to curves which are selected prior to making cloth Panels here...
I'm also assumimg this is the error eminem9144 is dealing with:
// Error: Curves must form a closed loop. Check for unconnected or extra curves. //
If this is the case then what I stated before holds true. It has nothing to do with opening/closing splines themselves. You cannot make a panel from less than two curves anyway, and you cannot use "closed" curves (in the sense to which you refer) in cloth - The "closed loop" is actually a series of two or more curves (on one plane) that meet each other end to end to form the panel outline.
If my assumptions are wrong then forgive my rating 