Of the many (pulp fiction) books I read during my teens, none made such a lasting impression on me as The Shockwave Rider by John Brunner. His description of a wired planet and computer worms inspired early experiments at Xerox Parc. Here's a quote from the book:

He sent a retaliatory worm chasing Fluckner's. That should take care of the immediate problem in three to thirty minutes, depending on whether or not he beat the inevitable Monday morning circuit overload. According to recent report, there were so many worms and counterworms loose in the data-net now, the machines had been instructed to give them low priority unless they related to a medical emergency.

The only successful commercial use of worms for distributed computation I'm aware of, is the illegal use of botnets to send spam. Frederick B. Cohen showed some interesting use cases for mobile code, though he seems to have abandoned research in this area after finishing his PhD thesis.

Recent developments:

• MOSREF and Mosquito Lisp: Mosquito is a secure remote execution framework available for download via the LGPL that combines high-grade cryptography and a small efficient virtual machine on both ends to ensure that intellectual property is protected. It also presents a dynamic environment on a target host that can be reprogrammed on the fly over a secure communications channel to fit the current situation.

References:

• A Short Course on Computer Viruses by Frederick B. Cohen: First concise theoretical work on viruses and mobile code.
• The Little Black Book of Computer Viruses by Mark Ludwig: First publication of working disassembled virus code.
• The Giant Black Book of Computer Viruses by Mark Ludwig.
• The Worm Programs: Classic analysis of the Morris Worm by Eugene H. Spafford.
• The "Worm" programs - early experience with a distributed computation: Early experiments at Xerox Parc.
• You Got the Power: Howard Rheingold's article about the first attempts to commercialize distributed computation.
• How to 0wn the Internet in Your Spare Time: Impressive analysis of the Code Red I worm.