Several library classes have diagnostic information to print at various levels of verbosity. Select a flexible but easy-to-use logging framework and use it uniformly throughout the library.

Looks like the common practice is to use a trait, acting as a facade to a standard Java logging library, what do you think?

See for example http://johlrogge.wordpress.com/2009/06/27/loggingtools-in-scala/ or http://github.com/davetron5000/shorty/blob/master/src/main/scala/shorty/Logs.scala

In any case, I would use slf4j (over log4j)