Solved: Installing Gems from Behind a Firewall
For a few years my team has been happily using Ruby and JRuby in our work for a big international bank. Suddenly, we could no longer install or update gems from behind the firewall. We eventually found that the reason for this was that our friendly firewall has decided to block all traffic to and from Amazon S3. This normally wouldn't be a problem but RubyForge and GitHub recently decided to let Gemcutter.org handle gem hosting and Gemcutter happens to use Amazon S3 to store gems.
If you're facing the same problem, I encourage you to chime in on this thread and let Gemcutter know.
Update (January 9, 2010): As of last night, the folks at gemcutter just completed the transition to using Amazon's CloudFront and in the process solved this issue.