Take a look at this program: http://pqrs.org/macosx/keyremap4macbook/
You can map any of your keys to any other keys, and there are several Vim-emulation settings. I'm not a hardcore Vim user so I can't vouch for the quality or anything, but I've seen some with a lot of work put into them.
About using KeyRemap4Macbook to get Vim bindings throughout the OS, follow this link and search for "Ubiquitous Vim Bindings"
http://pqrs.org/macosx/keyremap4macbook/list.html
The list of supported commands looks pretty solid, with the whole command / insert mode thing going on and certain commands switching modes automatically for you just like Vim.