Mike, unless I am missing something, that would require changing the method count() in the Counted interface to return a long, and that in turn requires little changes throughout the code base wherever Counted is used. It could be done, of course, but it is not a small change.
I agree that Mike's approach is nicer overall, but think Andy's patch is an immediate improvement over what we have now, and could be implemented until someone takes the time to correctly make all the detailed changes Mike is suggesting.
FWIW I did apply the patch, build, and test manually:
user=> (count (range (* 1000 1000 1000 3)))
ArithmeticException integer overflow clojure.lang.RT.countFrom (RT.java:549)
Perhaps of interest, Java's Collection.size() returns Integer.MAX_VALUE if the size of the collection > MAX_VALUE. I can't say that either that behavior or overflow is particularly helpful in practice of course.
Dupe to (similar change logged at Rich's request)