Drew Yeaton posted to the vim_mac group about an icon he created for MacVim:
After using MacVim for quite some time, I thought I would give back to the MacVim community in the form of design work. I'm proposing an alternate icon that I feel makes MacVim appear more at home on the Mac.
He posted the icon to Dribble, and has attached an icon file so you can use it with the MacVim app on a Mac. He also posted the original Photoshop document so it could be easily customised if required. Interestingly, Drew mentioned that one of his design goals was to keep in line with Apple's HIG, so it looks consistent alongside other icons in the Dock.