Gemcutter.org as a RubyGem source scares me
stream.btucker.org
stream.btucker.org
So no, you can't just upload a gem titled "rails" to Gemcutter and have it override the Rails team's gems.
This doesn't seem like a new problem to me. If this concerns you, just edit the sources list of your .gemrc file manually and make sure RubyForge is at the top and Gemutter is at the bottom.
More details on what I want to do: http://rubyforge.org/pipermail/rubygems-developers/2009-Augu...
The short answer is: be explicit with what source you're looking at, and start looking into gem signing/certs since it's the way we can trust gems.
More info here: http://litanyagainstfear.com/blog/2009/10/09/on-gem-forking/
*Edit: Looks like rufo already cleared this up...thanks man!