The eventual goal is for PuTTY to be a multi-platform program, able to run on at least Windows, Mac OS and Unix.
PuTTY has been gaining a generalised porting layer, drawing a clear line between platform-dependent and platform-independent code. The general intention was for this porting layer to evolve naturally as part of the process of doing the first port; a Unix port has now been released and the plan seems to be working so far.