I also failed to understand the "everything is a java.lang.Object" sentence. It doesn't make a clojure map mutable. In the end it's all just ones and zeroes, but that misses the point.
Ya totally, but I think OP was hinting at a fundamental tension between this benefit of interop and the desire to be functional where Java is (generally) not. Clojure lets you reuse the wonderfully huge library of Java solutions -- but those solutions are (generally) not written with immutability in mind, which I could see becoming an issue when you're trying to write a concurrent app. I suspect that's what the java.lang.Object reference was about, although obviously I'm not sure.
>Many wrappers hide mutability and provide an immutable API. And more java libraries are being written with immutable classes.
All that sounds encouraging....