Aaron's roughly right. We use C only for kernel work (and other maintenance on 3rd party stuff written in C) so I wouldn't count that as "application development", and Objective C for the very specific case of apps running client-side on Apple gear, etc.

Ruby is the embedded scripting language for Google Sketchup, see http://code.google.com/apis/sketchup/docs/gsrubyapi_examples.html -- that decision was made before Google acquired "@Last Software", Sketchup's makers.

Regarding Nishant's second question, in the wider job market, Ruby's kind of OK: still low absolute numbers but good growth, see http://duartes.org/gustavo/blog/post/programming-language-jobs-and-trends and http://blog.timbunce.org/2008/02/12/comparative-language-job-trend-graphs/ -- the data are getting a bit long in the tooth, but it's really hard to do these assessments in a very up-to-the-minute fashion;-).