For the Whyday, I've hacked two ZSH scripts for completion of RVM and bundler 1.0. You can find them on my dotfile repository on github: http://github.com/nono/dotfiles/blob/master/zsh/Completion/_rvm and http://github.com/nono/dotfiles/blob/master/zsh/Completion/_bundler.
To install them, you can put these 2 files in $HOME/.zsh/Completion (create this directory if it doesn't exist), and add it to fpath by adding this line to your .zshrc:
fpath=(~/.zsh/Completion $fpath)
A quick example of completion:
% bundle install --[TAB][TAB]
--binstubs -- generate bin stubs for bundled gems to ./bin
--deployment -- install using defaults tuned for deployment environments
--frozen -- do not allow the Gemfile.lock to be updated after this install
--gemfile -- use the specified gemfile instead of Gemfile
--local -- do not attempt to connect to rubygems.org
--no-color -- disable colorization in output
--path -- specify a different path than the system default
--quiet -- only output warnings and errors
--system -- install to the system location
--without -- exclude gems that are part of the specified named group