MythTV is controlled by an infrared receiver driven by LIRC, and it can also control my Panasonic Sky Digibox through an infrared LED.
The IR tranceiver consists of the basic homebrew transmitter and receiver described on the LIRC site, built onto a piece of 10mm x 15mm stripboard and stuck onto the front of the sky box.
The Digibox is very picky about the IR signal it receives, so here is a hand-tuned LIRC config file. Thanks to Stephen Davies for tuning most of the buttons, I did the rest. The scripts I wrote to send the IR signals to change channel are here. The skychannel script does the actual channel changing. There is also skybackupsend which sends the backup button press every 5 seconds to the Digibox to kill the annoying "interactive" overlay graphics that Sky insist on displaying over the screen. There is some primative locking to make sure the two scripts don't try to send signals at the same time.
I'm using the Sky Navigator remote to control Myth - it's designed for Sky+, so has the usual PVR buttons on it, and the top flips open to show a full QWERTY keyboard. There are quite a few buttons to set up, so here are my lirc.conf and lircrc files.