I like a lot the way Yi maps \ and -> to appropriate extended characters for display but leaves them in ASCII in the underlying file, but I didn't find it to be stable enough for primetime yet.
If you are using Vim >= 7.3, you can use the vim conceal feature which works as described, but without any messy hacks!
For Haskell, here ( https://github.com/vim-scripts/Haskell-Conceal ) is a plugin that works quite well, although the lambda symbol does not seem to appear for me, and the resulting glyphs don't flow nicely with the rest of my font. It's still kind of nice!
