Four years ago, a blogger named Kyle blogged about a simple but effective web script that allows most smart phones to control a MythTV setup.  Though he built his script for MythDora, it works wonderfully on MythBuntu with the following steps as root (assuming you have Linux experience):

• Grab: http://kylestoneman.com/out/remote.tar.gz and move its parts to necessary locations:

wget http://kylestoneman.com/out/remote.tar.gz
tar ‑zxf remote.tar.gz
mv remote /var/www
cp remote /var/www/remote/remote.py /usr/lib/cgi-bin

• Turn on the remote control — From MythTV Frontend: 
  • Go to Utilities/Setup -> Setup -> General
  • On the third page, check “Enable Network Remote Control Interface”
  • Make sure the “Network Remote Control Port” is 6546
  • Then exit and restart MythTV Frontend

• For extra credit, set up a nice icon for your iphone remote control (for the iPhone in case you add a bookmark to your home screen):

cd /var/www/remote
wget http://origin.arstechnica.com/journals/apple.media/64/mythtv_icon.png
mv mythtv_icon.png apple-touch-icon.png
cp apple-touch-icon.png /var/www 

Now you should be able to go to http://mythtv.local/remote/ (replacing mythtv.local with your mythtv machine name) on your smart phone and control your mythtv box…

Have fun!