Hey, Natobasso... your response was right on.
I'd manually checked for duplicates of Helvetica, which was the font that seemed to be causing the trouble, but hadn't found any. I already use Suitcase Fusion which came with Font Doctor, so I just scanned for duplicates and found multiples of...
I've been having this problem in Mail: It seems that some messages sent in italic, specifically Helvetica Bold Oblique, show up as HBO gibberish. I just hit the reply button and then change the gibberish area to any other font and it's rendered normally. So, still not having sussed out the cause...