How to disable Safari's new "feed://" redirection for feeds?
Apparently, it used to be possible in Tiger/Safari 2.0 to disable this using defaults write com.apple.Safari DebugSyndicationEnabled -bool NO. (source) I cannot get this to work in Lion/Safari 5.1, however.

Your best bet seems to be to set the default feed reader (Safari → Preferences… → RSS) to something that gives you XML source.