well, good luck with this, im out

nice to see you like the idea

and yes it is intended to be used by ruby developers to ease the script installation for end users.(But it can be used to install anything in fact)
I just modified the winamp plugin example that ships with nsis into what you see

how to use:

1. download nsis: http://nsis.sourceforge.net/Download
2. put all your files you want to release into one directory
3. download that installer script into that direectory and rename to installer.nsi
4. modify the file installer.nsi with a text editor. Things you should change:

Line 7: The name of the software you want to install. (Add version information):

Name "Ogre - Sketchup Plugin 1.1.0"

Line 53: What files to install:
change the File command (see here for more info)

  File *.rb
  File lesser.txt
  File readme.txt
  File ogre_export.htm

if you want to change the output path you can use this function.

  SetOutPath "c;\"

Line 73: The Instruction Website that is opened after the installation:

  StrCpy $0 "http://wiki.rigsofrods.com/index.php?title=Sketchup_Plugins#introduction"

Line 10: Determines how the result executable will be named (not such important):

OutFile "ogre-skechtup.exe"

5. after modifying the file, right click on it and select "Compile NSIS Script"

i hope that helps
nsis has a lot capabilities, so lots of possible ideas for extensions...

EDIT:
Juju: the Nsis system can create categories for installable content, like you know "install full" or "install minimal" or select single categories, so everything possible

also if you want to adopt this for all ruby plugins, we should use a common logo and icon, so the user recognizes it

i created a nullsoft installer for sketchup plugins:

it detects the sketchup installation directory and downloads sketchup if its not installed

sources: the nsi installer source. rename to .nsi

should be easy to modify for your needs