I've been making an effort to learn something about soft-serve, but there is little written about the history of it. I'm surprised, because I think it's an interesting bit of Americana. Mr. Softee's official website is a decent resource, but it may be a little biased. I've always found their ice-cream-brain mascot to be really creepy; probably even more terrifying than the Jack-in-the-Box monster. There's a short film called Swirlee which stars David Caruso as the roommate of a man with an ice-cream head. Let me give you the plot synopsis from IMDB:
"A city dwelling man with a giant ice cream cone for a head (who) tries to commit suicide by melting in a warm bath."
Pretty damned brilliant. The movie is terrible, but that's not the point. Here's the trailer, if you can endure it.
I bet you didn't know that the Mr. Softee song has lyrics, huh? I'll let you draw your own conclusions about them. This image is borrowed from their site.