please install libyaml and reinstall your ruby.

/Users/rupert/.rvm/rubies/ruby-1.9.3-p0/lib/ruby/1.9.1/yaml.rb:56:in `<top (required)>':
It seems your ruby installation is missing psych (for YAML output).
To eliminate this warning, please install libyaml and reinstall your ruby.

~% rvm remove ruby-1.9.3-p0
Removing /Users/rupert/.rvm/src/ruby-1.9.3-p0...
Removing /Users/rupert/.rvm/rubies/ruby-1.9.3-p0...
Removing ruby-1.9.3-p0 aliases...
Removing ruby-1.9.3-p0 wrappers...
Removing ruby-1.9.3-p0 environments...
Removing ruby-1.9.3-p0 binaries...

~% rvm list
rvm rubies
 
   ruby-1.8.7-p302 [ x86_64 ]
   ruby-1.9.2-p0 [ x86_64 ]
=> ruby-1.9.2-p180 [ x86_64 ]

~% rvm install ruby-1.9.3-p0
/Users/rupert/.rvm/rubies/ruby-1.9.3-p0, this may take a while depending on your cpu(s)...
 
ruby-1.9.3-p0 - #fetching 
ruby-1.9.3-p0 - #extracting ruby-1.9.3-p0 to /Users/rupert/.rvm/src/ruby-1.9.3-p0
ruby-1.9.3-p0 - #extracted to /Users/rupert/.rvm/src/ruby-1.9.3-p0
ruby-1.9.3-p0 - #configuring 
ruby-1.9.3-p0 - #compiling 
ruby-1.9.3-p0 - #installing 
ruby-1.9.3-p0 - updating #rubygems for /Users/rupert/.rvm/gems/ruby-1.9.3-p0@global
ruby-1.9.3-p0 - updating #rubygems for /Users/rupert/.rvm/gems/ruby-1.9.3-p0
ruby-1.9.3-p0 - adjusting #shebangs for (gem).
ruby-1.9.3-p0 - #importing default gemsets (/Users/rupert/.rvm/gemsets/)

~% rvm list
 
rvm rubies
 
   ruby-1.8.7-p302 [ x86_64 ]
   ruby-1.9.2-p0 [ x86_64 ]
=> ruby-1.9.2-p180 [ x86_64 ]
   ruby-1.9.3-p0 [ x86_64 ]

~% rvm use ruby-1.9.3-p0
Using /Users/rupert/.rvm/gems/ruby-1.9.3-p0

~/current[master]% gem list
 
*** LOCAL GEMS ***
 
rake (0.9.2.2)
rubygems-update (1.8.15)