http://www.redmine.org/projects/redmine/wiki/HowTo_Mylyn
http://danmunn.github.com/redmine_mylyn_connector/
https://github.com/danmunn/redmine_mylyn_connector
As of july 2012 now there is a new dedicated Redmine-Mylyn-Connector under active development for Redmine 2.x! (Got to admit, I haven't tested it yet.)
I am in the process of testing the 1.x connector that can be found on sourceforge, I will edit this post once I am done.
EDIT:
The update-site for the mylyn-plugin from the link in the accepted posting works.
As well as the mylyn-connector for the redmine installation. (Tested with 1.3.2 installation, don't forget to restart the web server after installing it. ;))
But both things aren't working out... once the mylyn-connector is installed, the web-interface is not working anymore. ;(
EDIT2:
The special connector works with 2.x and Mylyn finally looks proper.
http://danmunn.github.com/redmine_mylyn_connector/ the redmine server plugin.
http://kenjiru.ro/blog/2012/02/install-the-redmine-mylyn-connector-for-eclipse another server plugin link and the eclipse connector plugin
Will look like this.