Build a MU puzzle solver
SWI Prolog, 183 characters

m(A,A).
m([i],[i,u]).
m([i,i,i|T],B):-m([u|T],B).
m([u,u|T],B):-m(T,B).
n([m|A],[m|B]):-(m(A,B);append(A,A,X),m(X,B)).
n(A,B):-m(A,B).
s(A,B):-atom_chars(A,X),atom_chars(B,Y),n(X,Y).


How about some Prolog, (since nobody has answered in 6 months).
To run, just use "s(mi,mu)."  The code breaks up atoms into chars, then searches for the solution.