Redmine

After talking with DreamHost Support I got it working, I was 90% there.
http://wiki.dreamhost.com/Ruby_on_Rails#Transferring_an_Existing_Rails_App

I thought some other users getting stuck might find this helpful.
After the installation I needed to do the following steps:

1. Login into my DreamHost Shell Account
2. Run ruby redmine
3. Copy the dispatch.fcgi, dispatch.rb and for completeness dispatch.cgi from redmine/public/ created in the root of your account to the public folder of your actual Redmine installation
4. Change the permissions of the files you've just copied. chmod 755 public and chmod 755 public/dispatch.*
5. Got to your Redmine in browser, and all should be working.

/ Hami

Sorry - Step 2 is wrong, it should be:
2. Run rails redmine

/ Hami

To further this, you also need to replace the .htaccess file for the one that rail redmine creates.
The file should look like this:

# General Apache options
AddHandler fastcgi-script .fcgi
AddHandler cgi-script .cgi
Options +FollowSymLinks +ExecCGI

# If you don't want Rails to look in certain directories,
# use the following rewrite rules so that Apache won't rewrite certain requests
# 
# Example:
#   RewriteCond %{REQUEST_URI} ^/notrails.*
#   RewriteRule .* - [L]

# Redirect all requests not available on the filesystem to Rails
# By default the cgi dispatcher is used which is very slow
# 
# For better performance replace the dispatcher with the fastcgi one
#
# Example:
#   RewriteRule ^(.*)$ dispatch.fcgi [QSA,L]
RewriteEngine On

# If your Rails application is accessed via an Alias directive,
# then you MUST also set the RewriteBase in this htaccess file.
#
# Example:
#   Alias /myrailsapp /path/to/myrailsapp/public
#   RewriteBase /myrailsapp

RewriteRule ^$ index.html [QSA]
RewriteRule ^([^.]+)$ $1.html [QSA]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.*)$ dispatch.fcgi [QSA,L]

# In case Rails experiences terminal errors
# Instead of displaying this message you can supply a file here which will be rendered instead
# 
# Example:
#   ErrorDocument 500 /500.html

ErrorDocument 500 "<h2>Application error</h2>Rails application failed to start properly" 

You also need to repeat this whole procedure when you upgrade Redmine.

/ Hami

It looks like you have to login to your Dreamhost control panel and make sure you have fastcgi enabled for the domain as well.