I just got the Highlight about 1 month ago.  It was randomly vibrating during calls where I am not using my headset.  Rather I am holding the phone to my ear.  I researched the issue online, called Samsung and T-Mobile but could not get any answers about why this was happening.  I decided the phone was defective and had T-Mobile send me a new one.  I just received it today and while making calls without using a headset the new one does the same thing.

Has anyone else experienced their phone just randomly vibrating during calls for no apparent reason?     

I've looked at the minute minder possibility and that isn't it.  It doesn't vibrate in any sort of pattern.  It's completely random.  Once while talking on the phone in my vehicle (again, this only happens while NOT using a headset) I noticed that while turning my head from the right to the left it vibrated.  So I turned my head in the same way again and it vibrated again.  I did this about 8 times in a row and received a short vibration each time.  It was examples like this that caused me to initially believe that it was possibly an alert of reception changes.  However, I can find no documentation to back that up.
This is my second Samsung Highlight that randomly vibrates and neither T-Mobile or Samsung themselves seem to know why the phone is doing this.