Ah! So it wasn't just me or my imagination

Thanks for clearing that up.
Your iMovie suggestion worked perfectly -very easy!
But how do I save the file in the same MP4 format after doing that? I was looking for an "Export" option but couldn't find one. All I can see is "Share" with various options where "File" seemed most appropriate, but apparently only saves it as an iMovie project.
All I want to do is open the MP4 file, remove the audio, then save it again (preferrably without any loss or bloating the file-size). Are there any other apps which allow me to do this kind of thing now that my QuickTime player appears to be somewhat limited?
UPDATE: iMovie
does in fact save the file as MP4, and not as an iMovie project file as it first seemed to!
It takes forever to save the new file though (I had expected it to do it in a couple of seconds, but I assume it has to somehow process a completely new file). Around 14 minutes to create an audio-free MP4 of a 45 MB MP4 file on my mid-2010 Mac Pro with a 6-core 3.46GHz Xeon processor/24GB RAM. Does this sound right?
Also, the file size of the non-audio MP4 has grown to 102 MB -more than twice its original size!

I did compare the two files using QuickTime Player's "Inspector" function afterwards and they both have the same format (1920x1080), but data rate had increased from 193 kbit/s to 451 kbit/s and FPS changed slightly from the original 30 to 29.97.
Finally the "Current size" had changed from 512x288 to 572x322.
It would be great to do this in a much simpler (and preferrably quicker if possible) way -maybe an OSX service or something? Right click on a video file, select the service and have a copy of the same file made but without audio.
UPDATE 2: I've just read through
the Apple forum discussion you initially referred to, and it indeed suggests a (free as far as I can see) separate app for separating audio called
Subler. I'm going to give it a go and also look into the Automator action suggested in the Apple forum thread.
UPDATE 3: Yup,
Subler works! The documentation isn't very helpful for non-geeky types IMHO, but this appears to be the way to do it:
1) open the video file in Subler
2) select (highlight by clicking once on it) the audio track
3) press the BACKSPACE key to remove the audio track
4) save the file (or use "Save as" if you're working on the original in case you don't want to change that file but instead make a new copy)
I haven't yet looked into creating an OSX Service for doing all of this in one simple step.. if someone already has a working solution for that , please do share!