Ubuntu rails install fails on zlib

I've just moved over to Ubuntu 8.10 as my dev box; it's my first serious foray into Linux as a daily-use OS, and I'm having a hard time getting Rails going. I have followed a number of tutorials which all seem to work fine, but when I try and use gem install or gem update on anything, I get an error that looks like this: /usr/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `gem_origi