Of course, the correct answer is (IIRC) that standard Java web servers don't support the NIO asynchronous request paradigm - the standard Java servlet spec is thread-per-request.
But not everyone knows that.
Jetty has also supported their own async handling extension for quite some time now (but they're moving to the 3.0 standard). I've used it to build 'Comet' servers before, and it does the trick.
In that case, someone should write a Clojure library that utilizes the 3.0 standard. That'd be even cooler.
I think Netty is great if you want fine grained control over each aspect of your network stack and/or ultimate performance. But having a full featured web server that does sync and async equally well is a value that shouldn't be dismissed either.