This would require emulation of the Microsoft XNA Framework (or whatever it's called). Porting something that uses Microsoft dependencies onto a platform without them would require a lot of recoding I imagine as a new API that is linux supported would have to be used, and oh dear gawd entire core rewrite (probably).
If osu! were to ever become playable on Linux, I imagine it'd be the Linux community's doing, upgrading of WINE or other windows emulation software, rather than peppy's doing, ie rewriting the program.
I believe this has been brought up before and what it boils down to is, the effort required on peppy's behalf (rewriting osu! with a new API that is Linux compatible) is much greater than the effort required on the rather small demograph of Linux users' (dual-booting Windows). I run Linux, but I take the latter approach (well in reality I don't use my dual boot, I just have one machine that is Windows and one that is Ubuntu).