Kaspar Schiess has just uploaded an alpha version of RMagick 1.4.0 for
Windows. This is for Ruby 1.8.1 and 1.8.0.
Check it out! Thanks, Kaspar!
P.S. The doc is not included but you can use the online version at
http://www.simplesystems.org/RMagick/doc or
http://studio.imagemagick.org/RMagick/doc.