If you just type "@1" is the repl, on previous versions you'll get an error that Long cannot be cast to IDeref. In 1.5, the error message is that it cannot be cast to java.util.concurrent.Future. This is because it assumes that anything that isn't an IDeref is automatically a Future. The deref method should generate a custom error stating that the type you've passed in is neither an IDeref nor a Future.
Found this on ubuntu, lein 2, Clojure 1.5 snapshot, but it's obvious from inspection
Attached a patch that implements the old behavior (can't cast to IDeref), which strikes me as good enough considering the support for j.u.c.Future seems rather an edge case (being that clojure's futures are themselves IDeref).
The weirdest thing I did was to use clojure.core/cast to unconditionally throw a ClassCastException. Let me know if that's weird and I'll do something different.