I'm able to encode TrueHD, but for whatever reason this format isn't commonly used and may cause compatibility problems of its own.
For creating AC3, by far the easiest way to get access to a Dolby-certified AC3 encoder is with the Compressor application from Apple. You'll get the superior sound quality the official algorithm offers as well as a host of video options, all quite affordably. I've compared its output to that from Dolby Media Encoder and they are quite indistinguishable at a given bitrate as far as I can tell.
One of the things I like about using Compressor for AC3 is that it can encode straight from Apple Lossless files without having to go to WAV or AIFF first, which is useful for me because I often archive things in Apple Lossless to save hard drive space. Compressor does, however, have the drawback of defaulting to a DialNorm value of -24, which equates to a 7 dB reduction in volume, so you have to remember to set it back to -31 if you want your levels to remain unaltered. By contrast, Dolby's own encoder now starts at -31 instead of -27 the way it apparently did in the past.
Both programs have a setting for -3 dB surround channel attenuation, which is used for lowering the rear channel levels from theatrical to home calibration standard and is necessary for most 5.1 film mixes when transferring to home video. This is not needed for any mix that I have created, and so should be unchecked—otherwise the rear channels would come out too low. Similarly, the setting for 90-degree surround channel phase shift should be unchecked for my stuff, since I already accounted for such things when putting the 70mm version together.
I guess I never did actually mention it here, having only talked about it sporadically in other threads, but there is indeed a new version of the 70mm mix out there. The changes from the previous revision are minimal, with the main improvement coming from having used the Dolby Media Decoder application to upmix the laserdisc audio into five channels with Prologic II, which results in greater channel separation and consequently a more believable sound field. (The better the acoustics of your room and the more accurate the sound system, the more likely it is that you'll notice the improvement.) I also changed the level of one bass effect in the LFE channel so that it would blend better with the rest of the mix, but everything else is exactly the same as before. The other two films will eventually receive the same treatment.
I'll be posting in more detail about this and about optimizing the other audio tracks later on, so watch for that if you're interested. Links to all the new files I've made will be provided.