In a nutshell, give your grammar files a .ym extension instead of .y. Xcode will then run Bison with the necessary magic to support Objective-C.
Oddly enough, I found this question because I wanted to do the same thing. I remembered a discussion about it on Apple’s cocoa-dev mailing list, but this question came up higher in my Google search. I found the discussion I remembered though, on CocoaBuilder – there’s a pretty extensive example at the end. I hope it helps both of us, LOL!
(Edit) Beats the heck out of me how anyone ever managed to find this out, by the way – I still haven’t found any official documentation about it.