For years I've heard the argument that Mathematics should be Maths (being Australian). That's the first time any one has made a sensible argument (to me) for the alternative.
I've never heard anyone call Gymnastics "Gyms". :P
Interestingly, in Spanish and French the unambiguously plural terms 'las matemáticas' and 'les mathématiques' are commonly used, so the 's' in 'mathematics' may be of a different kind than that in 'gymnastics' or 'economics'.