I installed ruby using rvm on Ubuntu. Check both rvm current and ruby -v to make sure 2.0.0 was being used.

Here is the error:
Code:
mikeb@ubuntu:~/projects/sinatra$ ruby hello.rb
/home/mikeb/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:51:in `require': cannot load such file -- sinatra (LoadError)
	from /home/mikeb/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:51:in `require'
	from hello.rb:1:in `<main>'