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As of mid September '06 it can build ASTs and tree parsers are working. 
The next ANTLR beta release will include a working Xcode plugin, so you can automatically generate your grammars from within Xcode.
It automatically tracks all the dependencies between the different grammars in your project to build them in the right order.
For the moment it doesn't pick up the Common error messages or warnings from the ANTLR tool, but that's on my list. See belowand warnings are picked up for display in Xcode, but we really need to clean up ANTLR to generate more easily parseable messages. I'm working on that.
ANTLR.framework
Features (partially) missing are:
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