In looking around the web a bit, I've had a hard time finding any capistrano plugins that handle deployment of SVN tags and branches well. So, I wrote a little one of my own.

Capistrano.configuration(:must_exist).load do

  set :svn_trunk_dir, 'trunk'
  set :svn_tag_dir, 'tags'
  set :svn_branch_dir, 'branches'
  
  before "deploy" do
    if rails_env == 'production'
      abort "ERROR: Production deploys must be from a tag or branch." unless repository =~ /#{svn_tag_dir}|#{svn_branch_dir}/
    end
  end
  
  desc <<-DESC 
  Deploy from a tag instead of from the trunk. This will prompt if no TAG value 
  is specified on the command line. No sanity checks for whether the tag actually
  exists are performed.
  DESC
  task :deploy_tag do
    set :repository, get_svn_url(svn_tag_dir, 'TAG')
    run_deploy
  end
  
  desc "Create the specified tag and deploy from that tag."
  task :deploy_new_tag do
    trunk = "#{repository[/(.*?)(?:\/trunk)?$/, 1]}/#{svn_trunk_dir}"
    set :repository, get_svn_url(svn_tag_dir, 'TAG')
    puts "Creating new tagged release of trunk as #{ENV['TAG']}."
    system "svn cp #{trunk} #{repository}"
    run_deploy
  end
  
  desc "Deploy from a branch instead of a tag; same caviats as for :deploy_tag."
  task :deploy_branch do
    set :repository, get_svn_url(svn_branch_dir, 'BRANCH')
    run_deploy
  end
  
  desc "Create a branch from the specified tag and deploy from that branch."
  task :deploy_new_branch do
    tag_url = get_svn_url(svn_tag_dir, 'TAG')
    branch_url = get_svn_url(svn_branch_dir, 'BRANCH')
    puts "Creating new branch #{ENV['BRANCH']} from tag #{ENV['TAG']}."
    system "svn cp #{tag_url} #{branch_url}"
    set :repository, branch_url
    run_deploy    
  end
  
  # Executes the ordinary deploy task
  def run_deploy
    puts "Deploying using repository url #{repository}"
    execute_task(find_task("deploy"))
  end
  
  # Prompts the user for a tag and updates the repository url based upon that
  # tag and the svn conventions.
  def get_svn_url(tag_dir, env_var = 'TAG')
    while ENV[env_var].nil? || ENV[env_var].strip.empty?
      ENV[env_var] = Capistrano::CLI.ui.ask("Please specify #{env_var.downcase} name: ")
      ENV[env_var].strip!
    end
    
    repo_root = repository[/(.*?)(?:\/trunk)?$/, 1]
    "#{repo_root}/#{tag_dir}/#{ENV[env_var]}"
  end
end

I just stuck all of this in lib/cap/tag_deployer.rb, then added

require 'lib/cap/tag_deployer.rb'

to my deploy.rb file, and away I go!