Hello,
if entering solve(floor (x)-5,x) in master, I get a NotImplementedError.
Wolfram Alpha gives the correct result (see http://www.wolframalpha.com/input/?i=solve%28floor%28x%29-5%2Cx%29 ): 5<=xɞ
I have started to implement floor.inverse(), to get a start.
I will commit this to a branch in my github clone today and give a link.
Most probably I will need some some support to implement complex numbers.

Original issue for #7074: http://code.google.com/p/sympy/issues/detail?id=3975
Original author: https://code.google.com/u/107445818890982593796/