This json gem without windows binaries for Ruby 1.9.2 is the bane of my existence.
I just started using heroku today with the heroku gem, and on every command I got the dreaded "msvcrt-ruby18.dll was not found" popup.
Edit: As pointed out by Chad Tipton in the comments, you'll need to add --version=1.4.6 because 1.5.1 is out but 1.4.6 is the version of the json gem that the heroku gem depends on.
> gem uninstall json > gem install json --platform=ruby --version=1.4.6
I also learned that there's a batch script to set your PATH for using DevKit in your DevKit install dir called devkitvars.bat that will add install-dir\bin to your PATH... that's what I get for messing around with my PATH for no good reason.
I really should just start running "gem install json --platform=ruby" after every command.