Whenever I used to get the Scala REPL warning message, "There were deprecation warnings, re-run with -deprecation", I used to have to restart the REPL with the -deprecation flag to see the warning message, like this:
$ scala -deprecation
That works, but sadly, you had to lose your entire REPL session to run it.
Use :warnings
Thanks to the Scala gods, this is no longer necessary in Scala 2.10 (and newer). Now, when you get the REPL deprecation warning message, like this: