I think it is an excellent idea to have a Ruby shell!
Instead of Bash, call it Rash!
It wouldn't be easy to create the thing, considering the other *nix  
shells are all pretty robust and well developed.
However, they're also pretty old and do things in ways that are not  
always fun or easy. If you use bash or another shell all day every  
day, then you get used to it just like C++ or Java, but that doesn't  
make it better.

It's ambitious, but irb makes it seem like a darn good idea!

Imagine a new distro or two of linux: Rinux!!
An Ubuntu derivative called Rubuntu!!
with a shell named Rash!
How could you go wrong with cool names like that!?