This is where PB apparently got his tip from:
http://www.cod3r.com/2008/02/the-correct-way-to-enable-ac3-passthrough-with-quicktime/

According to that blog, this does not work with Perian 1.1.4 due to a bug, to be resolved in the next version of Perian. So if it isn't working for anyone, that may be the reason. Oh yes... and they also say to use 2ch-16bit, not 24bit.

From my experience, what I can say is that any source material I have that has simply a Dolby Digital sound track works (and has always worked) just fine. The problem I'm seeing though is with iTunes purchases that have two tracks... AAC and AC3. My sound continues to mute when switching to AC3 in spite of following this tip, though no small surprise since the Perian developers themselves say it doesn't work as mentioned. So here's hoping the next version of Perian is imminent.

I mis-read the tip from the link I posted... it's DTS tracks that are buggy, not AC3 tracks.

I experimented with the tips here after realizing that iTunes really wasn't playing back my surround sound tracks, nor was Front Row. I've only recently started using FR on my mini for a media center, but before this I was using an Apple TV that played ss tracks fine, as does VLC and MPlayer Extended.

Despite my best efforts and trying everything I could think of and find tips for, it just doesn't work. Sometimes the ss track is mute, sometimes I get a loud "clacking" noise. I did a little investigating using one of my purchased HD episodes of Dollhouse, and I found that the AC3 track shows as disabled under the movie properties using Quicktime. THAT is just weird. The only solution I could come up with was to strip the DRM and then use Subler to strip the AAC track and enable the AC3 track. Oddly now, this fixed file plays fine in iTunes, but now Front Row doesn't recognize it. I've given up and switched to using Plex. Plex isn't exactly intuitive to set up (it took me half this weekend to sort it out), but is soooo much more useful than Front Row once you figure it out.

This is simply far more tedious than it should be. I'm utterly appalled over how badly Apple's own purchased shows are supported on their own Macs. It seems they are trying to push people into using the Apple TV to get access to the surround sound tracks, but this is far from transparent. I got rid of my ATV in the first place because it was so underpowered and so limited in features and media support, even after using a "patchstick" to try to get around its limitations. This really flies in the face of the notion that Apple's stuff "just works". I guess it does work, just not the way we think it should.