Flirc is a small usb device that plugs into your computer and pairs with your same television remote. There is a setup process to do the pairing and the software runs on Apple, Windows, and Linux.
The benefit with flirc is that you don't need an extra remote for your living room.
The device works by pairing remote buttons with keyboard keys. Given that all media center apps, including mythtv, use keyboard keys, there is no special steps needed to have your remote control the application. It will just work.
You can do advanced things like pairing a remote button with something like the following: shift+ctrl+F1. You can set up your OS to do something with this keyboard sequence, like restart mythtv, re-scan a directory, put your computer to sleep, etc.
FLIRC in the UK/EU
For all UK/EU residents, FLIRC is now available via http://www.mannmediacentres.co.uk/