It may sound insignificant but we migrated our most important
applications in production from log4j 1.3 over to logback version 0.7. We have done this without changing a single line of code in our applications but by merely replacing the file log4j.jar with log4j-bridge.jar in the relevant WEB-INF/lib directories.
Log4j-bridge intercepts calls to log4j's Logger class and transparently redirects them to logback.
I am thrilled to see that for our real-world applications previously using log4j, the migration process to logback was quick and painless.