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As of mid September '06 it can build ASTs and tree parsers are not far off anymore 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 error messages or warnings from the ANTLR tool, but that's on my list. See below.
ANTLR.framework
Features (partially) missing are:
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- Single rule return values
- FOLLOW sets (bitsets)
- cyclic DFAs
- Scopes
- dynamic scopes
- rule parameters
- Labels
- token reference labels
- Syntactic Predicates
- Tree support (buildAST=true)
- Tree parsers
Xcode plugin
Features missing:
- Warnings & error messages in Xcode (might need to change ANTLR a bit for that)
- Syntax highlighting & function popup support (I'd still encourage you to use ANTLRWorks for development!)
- Debugging (long term goal)
Features completed:
- Build settings you can pass to org.anltr.Tool
- Dependency calculation
- Code generation
Roadmap
Building ASTs and tree parsers is working now. ANTLR.framework is known to be Universal.
I will use v3 to write a number of different small applications in order to find out about all the small bugs that are hiding in the corners. Thus development will probably not be milestone based, but rather fix-as-they-come-up.
Regarding the template output support of v3, this will require an Objective-C implementation of ST, which will take a lot of time. So don't expect that for 3.0.
Soon I will add debugging support to the code generation and add a bit to ANTLRWorks to make it support debugging Objective-C based grammars in their natural environment (=your application) or as simple standalone recognizers.
There is an initial Xcode plugin available in the depot.
Platforms
I will only concern myself with the Mac. There will be no GNUStep support.
As for Mac OS X versions, I aim to be compatible with at least 10.3 as a target platform, though the Xcode project will require the latest version available, since it really doesn't make a lot of sense to use it in an earlier version.
The Xcode plugin requires Xcode 2.3 but should run with 2.4, too.
Currently ANTLR requires 10.4. Unless there are very good reasons to support versions prior to that, it will not get priority.
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The goal is to provide the complete set of Java-based examples in Objective-C for comparison. There may be additional examples contained in the Xcode project. These exist mainly for development purposes as testcases, or to try out features.
I am currently working on an editor for StringTemplates as a big "example"/testcase for v3.
Also take a look at the Xcode plugin that uses a filtering lexer written using ANTLR to figure out grammar types and dependencies.