Pito Salas wrote:
> Any recommendation on which 'gui' package I should use to build a ruby
> client application?
> 
> Here are some of my needs, some of which have bearing on the
> package I choose:

I think wxRuby meets all your requirements, and would be worth 
considering along with other options. Some extra comments below:

> - Should run on windows, mac and linux
> - I don't have a strong requirement that the app looks super 'native'
> on each platform.
> - I should be able to create a simple installer for it

If you model your wxRuby application on Daniel Zepeda's template, you 
get rake tasks for building appropriate installers for OS X and Windows.

http://daniel.zepeda.ws/blog/2009/04/18/announcement-wxrubytemplate/

> - User should be able to launch it from the desktop
> - The UI is relatively simple: menus, windows, dialog boxes
> - A key part of the interface is a graphical display of a scanned
> image (from a .tiff for example) and allowing the user to use the
> mouse to drag out rectangles annotating the image

I wrote a wxRuby class that does just this (and panning / zooming of 
images) as part of an application:

http://weft-qda.rubyforge.org/svn/trunk/weft-qda/lib/weft/wxgui/controls/image_viewer.rb

> - In a different command, the graphical display of a scanned images
> needs quickly cycle through one after the other
> - The graphical display of the scanned image needs to be zoomed in and
> out and panned with the mouse

wxRuby has a pretty strong Wx::Image class which works with a variety of 
formats (JPG/TIF/BMP/PNG/TGA) and supports a variety of transforms 
(rotate/scale/flip etc)

a