Looks like documentation is not installing properly for Rails3 beta3.
Here is the error I'm facing :
Successfully installed rails-3.0.0.beta3
1 gem installed
Installing ri documentation for rails-3.0.0.beta3...
ERROR: While executing gem ... (Errno::ENOENT)
No such file or directory - lib
debian:~# gem install rails --pre
Successfully installed activesupport-3.0.0.beta3
Successfully installed activemodel-3.0.0.beta3
Successfully installed actionpack-3.0.0.beta3
Successfully installed arel-0.3.3
Successfully installed activerecord-3.0.0.beta3
Successfully installed activeresource-3.0.0.beta3
Successfully installed actionmailer-3.0.0.beta3
Successfully installed railties-3.0.0.beta3
Successfully installed rails-3.0.0.beta3
9 gems installed
Installing ri documentation for activesupport-3.0.0.beta3...
Installing ri documentation for activemodel-3.0.0.beta3...
Installing ri documentation for actionpack-3.0.0.beta3...
Installing ri documentation for arel-0.3.3...
Installing ri documentation for activerecord-3.0.0.beta3...
Installing ri documentation for activeresource-3.0.0.beta3...
Installing ri documentation for actionmailer-3.0.0.beta3...
Installing ri documentation for railties-3.0.0.beta3...
Installing ri documentation for rails-3.0.0.beta3...
Installing RDoc documentation for activesupport-3.0.0.beta3...
Installing RDoc documentation for activemodel-3.0.0.beta3...
Installing RDoc documentation for actionpack-3.0.0.beta3...
Installing RDoc documentation for arel-0.3.3...
Installing RDoc documentation for activerecord-3.0.0.beta3...
Installing RDoc documentation for activeresource-3.0.0.beta3...
Installing RDoc documentation for actionmailer-3.0.0.beta3...
Installing RDoc documentation for railties-3.0.0.beta3...
Installing RDoc documentation for rails-3.0.0.beta3...