A while back, I noticed KMail was starting to run really slowly, and was responsible for a few megabytes per second of disk IO (as reported by iotop). I did some google searching, and found a reference to a possible problem with importing large folders from older versions of KMail. Since this was an upgrade (form KMail in KDE 3.5.x to KDE 4), I thought I would try cleaning out some of my old folders. I got part way through, and the disk IO did not let up. I did some more googling, and found this bug report: this bug report.
Reading through the bug report, I came across Message 22. It mentioned deleting the directory containing the saved searches. I did this, and the problem went away! I then remembered I had done a search over my entire message tree several days back. The saved search functionality keeps updating that search, so must traverse the message tree again and again, thus generating the very high disk IO.
I hope this solution helps someone!