kennyjensen says you're right about mixed states and superpositions being different:

i'm going to have to go with greenanemone on this one.  a mixed state is                                                                     
different than a superposition.  a mixed state describes an ensemble of                                                                      
states.  maybe 1/2 of the states in the ensemble are in the state |00>+|01>                                                                  
and maybe the other 1/2 are in the state |00>+|11>.  a superposition                                                                         
describes one state which could have components along a number of basis                                                                      
states.                                                                                                                                      
                                                                                                                                             
if you want to learn more about mixed states look up stuff on the "density                                                                   
matrix."  sakurai has info about it.  but actually, nielson and chuang, a                                                                    
quantum computing text, might be the best resource.             

But I think that 'superposition' does have the meaning I intended.