(This continues the discussion in this issue, which was centred on removing obstacles for newbies to learning and deploying Jekyll. Summarized, the discussion was to take responsibility for the whole “stack of installation”.)
To ensure nothing differs between development and production, the idea is to help users manage their gem dependencies and environment through
$ jekyll venv
An example of what happens when development and production have differing dependencies (gems) installed can be viewed in this example issue.
I think doing a "Getting Started with Jekyll and rbenv" like @parkr said will be really great and will solve the root problem in this case; anything more (in my opinion) like adding a virtualenv will be a mistake because its adding one extra thing to learn before you start working in what you want.
For your isolation case, Ruby already has Bundler and Rubygems, and running bundle exec before your commands will ensure nothing differs between your development and production gems.
@jekyll/owners: Can be closed. Now we have the Help repository https://github.com/jekyll/jekyll-help
Always open to reviewing a guide! Probably not that interested in baking virtual env control into Jekyll, but our docs could always use more info. :)