Well, really nice to finally have the mono button back (called Downmix Output now). 5.1 is something I've haven't experimented with to date, but it's great that this button converts stereo>mono too.
Yep, there's a lot of unpublished features and improved enhancements in Vegas 5. Keeps it interesting as you work, when you run across new things as you become more familiar with the new version.