Any news on this? Maybe plagin is available?
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There is a gem @redcloth-formatters-docbook@ which can do some basic conversion from textile to docbook.
Safe the TODO's which are easy enough to add conversion works quite nice.
So I thought I share this little program and maybe someone can integrate something in Redmine.

<pre>
# Convert textile formatted files to docbook
#

require 'optparse'
require 'fileutils'
require 'rubygems'
require 'RedCloth'
require 'redcloth-formatters-docbook'

revision = "0.9 - 2012-08-29"

# CHANGELOG
# 0.9 initial version

# Program options
@options = {}

optparse = OptionParser.new do|opts|
# Set a banner, displayed at the top
# of the help screen.
opts.banner = "Usage: tex2doc.rb [optionen] datei ..."

# Define the options, and what they do
opts.on( '-v', '--version', 'Print version' ) do
puts "tex2doc " + revision
end

# This displays the help screen, all programs are
# assumed to have this option.
opts.on( '-h', '--help', 'This message' ) do
puts opts
exit
end
end

# Parse the command-line. Remember there are two forms
# of the parse method. The 'parse' method simply parses
# ARGV, while the 'parse!' method parses ARGV and removes
# any options found there, as well as any parameters for
# the options. What's left is the list of files to resize.
optparse.parse!

puts "Tex2Doc Version #{revision}, Help: -h, --help" unless ARGV.size > 0

# TODO replace <pre> by <programlisting>
# TODO search empty table cells ||, fill with a single blank should do
# TODO escape <, >, &

ARGV.each do |file|
textile = File.open(file, 'rb') { |f| f.read }
docbook = RedCloth.new(textile).to_docbook
File.open(file + '.xml', "w") { |f| f.write docbook }
end
</pre>

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