We do the inverse. We tag first, then instruct Capistrano to grab the latest release (with a prompt). We have a release plan though, where software goes through QA before it's tagged. Only software ready for production gets a tag. Everything up to that point is done on a release branch.

    set :branch do
      default_tag = `git tag`.split("\n").last

      tag = Capistrano::CLI.ui.ask "Tag to deploy (make sure to push the tag first): [#{default_tag}] "
      tag = default_tag if tag.empty?
      tag
    end

Yeah, modify to taste. I only wanted to post some example code to show how easy it is to automate tagging.

My code won't be appropriate if you don't use master/staging/production branches, either.