My favorite writeup on logging is from the soon-to-be-released book "Growing Object-Oriented Software, Guided by Tests" by Steve Freeman and Nat Pryce, the authors of the JMock tool.  They've published an early draft online so you can read it now:

http://www.mockobjects.com/book/listening-to-the-tests.html

See the section "Logging is a Feature"

The short answer is that most logging tends to be an afterthought rather than treated with the same care as the other business-requested features.  This typically means a proliferation of log statements scattered throughout the code.  If the logs we're talking about are used by the operations group for critical support, then it's important that those messages get unit tested.  The most common use of Log4j sprinkles static method calls to Logger.getLogger() throughout several classes and is hard to test.  Instead, Freeman and Pryce recommend treating logging as a service which gets injected into classes that need it, and unit tested that way.

Structuring the logging in this way will make it much easier to write unit or acceptance tests.

Dan