After struggling with CEC adapters and various LIRC configurations, I bought a FLIRC for my parents to use with their MythTV setup.
Set up is a Mythbuntu HTPC, Logitech Harmony One, FLIRC, TV etc etc
I live overseas, so I sent them instructions on what to do (and set up the Harmony profile online for them so they just had to update it).
I used the Computer > HTPC > FLIRC > XBMC Harmony Profile, and mapped things that MythTV doesn't use (like switching zoom level, switching subtitles etc) for some more MythTV like functions (PIP, PIP control/swap, aspect ratio, switch tuner etc). I can't remember exactly what I used, but it won't matter for your setup.
The parents set up their FLIRC with their Windows 7 PC, using full keyboard mode. I sent them a subset of the key map found in the MythTV wiki, with a list of the Harmony buttons they should program for each key. They then unplugged the FLIRC and plugged it into the Mythbuntu PC, and reported back that it was working great! Good news for them (and less troubleshooting for me).
The FLIRC seems like a great product - I see there are a few questions about which Harmony profile to use on the forum - I would recommend using the XBMC one, and customising the buttons to your own individual needs, this gave a good result.