Convert a .doc or .pdf to an image and display a thumbnail in Ruby?

A simple RMagick example to convert a PDF to a PNG would be:

require 'RMagick'
pdf = Magick::ImageList.new("doc.pdf")
thumb = pdf.scale(300, 300)
thumb.write "doc.png"

To convert a MS Word document, it won't be as easy. Your best option may be to first convert it to a PDF before generating the thumbnail. Your options for generating the PDF depend heavily on the OS you're running on. One might be to use OpenOffice and the Python Open Document Converter. There are also online conversion services you could try, including http://Zamzar.com.