when installing radiant on a 1.9.0rc1 box:

gem install radiant
Building native extensions. This could take a while…
ERROR: Error installing radiant:
ERROR: Failed to build gem native extension.

/usr/bin/ruby extconf.rb
mkmf.rb can’t find header files for ruby at /usr/lib/ruby/ruby.h

Gem files will remain installed in /usr/lib/ruby/gems/1.8/gems/RedCloth-4.2.2 for inspection.
Results logged to /usr/lib/ruby/gems/1.8/gems/RedCloth-4.2.2/ext/redcloth_scan/gem_make.out

(from redmine: issue id 158, created on 2009-09-06, closed on 2009-10-02)