I highly recommend reading this article detailing how to manually flag commercials using the editor, and how to manually transcode your older recordings.http://www.mythtv.org/wiki/index.php/Removing_Commercials
In about 3 hours of time last night, I manually flagged commercials and transcoded about 100 old recordings (Ok, a lot of them were already commercial flagged, but I still checked through them for accuracy). The editor is a very simple and genious system that made doing this very easy. Also, for the lossless transcoding, it is very fast, taking only a few minutes per recording. I found the editor very fun and easy to use. All of my recodings now have no lead-in, and no commercials. Not to mention that I freed up almost 400GB of disk space.
Play any video in MythTV. Hit "M" to bring up the menu, and choose the Editor option. You will be brought into the timeline editor (a timeline bar will appear at the bottom). Hit the "Z" key to load the current commercial cutlist (if there is one). Now you can edit the existing cutlist for more accuracy, or start building your own. The controls are simple:
Up/Down Arrows - changes how much time you move along the timeline with each Left/Right Arrow press. (the current amount is shown in the bottom-right of the screen) This varies from 1 frame, to 1 second, 1 minute, 1 cutpoint, etc...
Left/Right Arrows - Move you along the timeline by the amount set by the Up/Down arrows
Enter - Cutpoint operation. You can insert a new cutpoint, move the closest existing cutpoint to this location, delete the closest cutpoint, etc..
Esc - Done
After you have the cutpoints, from the movie playing, hit "M" again to bring up the menu, Transcode->Profile (I select Default), and the transcoding job will be added to the Queue.
Again, once set up right, new recording will automatically flag commercial cutpoints and transcode each new recording. (i've found the accuracy to be quite good)