A shot at implementing a REPL-like feature for vim (and everything else able to write to text files).
Add the following to your ~/.irbrc:
require 'soma' Soma.start
When you start IRB Soma will listen on a file named /tmp/username_somarepl_buffer and execute everything added there in your IRB session.
(And for the sake of God, use Wirble for a prettier IRB!)
Copy plugins/soma.vim to ~/.vim/plugin/. The default mapping is Ctrl-c Ctrl-c, which sends the current paragraph or visual selection to IRB.
Quirks & Annoyances
Commands are added to IRB history, but you have to go one item back in your history or invoke some command before it's updated.