Wow, I thought for sure this would be one of those 5 minute turn around questions :)
The best answer I've come across comes from the Apress book Pro JavaFX 2.0 A definitive Guide to rich clients with Java technology. It contains a large section devoted wholy to Visage. It says:
Visage is an exception, inasmuch as it was originally conceived by Christopher Oliver as a UI language called F3 for Form Follows Function, specifically targeted at making it easier to program graphic user interfaces. It was renamed JavaFX Script and served as the UI programming language for all JavaFx applications prior to the release of javafx 2.0. Currently, it is being developed as an independent UI programming language called Visage, available as open-source, and can be used with JavaFX and other UI toolkits."
-- page 465, Chapter 10
Intriguing... I take this to be mean it is similar to, but not the same as, JavaFX Script and that it ought not be too hard to pick if you have FX Script knowledge.
This book, by the way, is a fantastic resource on JavaFX 2.0. I highly recommend it.