Besides the usual list of customer-driven bug fixes, there's lots of other added goodness, including updated JDBC drivers, web server connectors, Apache Multi-Processing Modules (MPMs) support on Unix, performance enhancements and improvements, JRun Web Server (JWS) - JWS keep-alive functionality, a new log level for HTTP requests, enhanced OS support (HP-UX 11 and 11i), Mac OSX 10.4, Solaris 10, AIX 5.3, RedHat 8, AS 2.1, AS 3.0, AS 4.0 and SuSE 8.
JRun 4 Updater 6 also adds support for JDK 1.5 (note that CF is not supported on JDK 1.5 yet, however).
The ColdFusion team has "blessed" JRun 4 Updater 6, so it's "good to go" for CF customers running CF on the full version of JRun 4.
ColdFusion customers can also rest assured that any security and other critical fixes included in JRun 4 Updater 6 have been brought back into the forthcoming CF "Merrimack" release (all install configurations), although Merrimack will be shipping with JRun 4 Updater 5+ fixes, since the timing of the two releases didn't permit us to include JRun 4 Updater 6 this time around in the installer kits.
Some useful links: