As I always wonder with Racket, I find myself wondering how much of this stuff is standard Scheme and how much is Gambit-specific. It would be lovely to program in a common subset fo the two, because I like Gambit, but Drracket is a very convenient development environment.
Gambit/Racket unification is not (yet) on my TODO. Gerbil has gone part of the way there, so maybe some of the Gerbil goodness should be ported back to Gambit so that all users can benefit.
Guile also very recently added a R7RS command line switch, so there is movement in the direction of greater standardization. The switch is in the master branch of Guile but AFAIK not yet in any published release.
The R7RS Large Edition standard is actively worked on, but I got the impression Racket is based on R6RS with plenty of custom extensions.