OK,
I heard that the Math on the GMAT are theorically on an "High School" level. So I searched on the web for Permutation and Combination problems and came accross this site:

http://mathforum.org/library/drmath/...rms_combs.html

Now, this is really for High School students, but they have some classical (and sometimes very hard) problems.
I have submitted a few of those problems on this forum.

Tell me what you think!