Well, I've just tried uploading the latest version of the firmware for my Creative Zen Vision:M, and now have a non-functional device.
First of all, the software that should have done the updating complained that it couldn't find the device, even though it was connected.
Could this be due to me running Windows Media Player 11 Beta, rather than WMP 10?
Stupidly, I then went into the recovery mode of the mp3 player to try and force it to update the firmware. Which meant I deleted the firmware from the player without then being able to upload the new firmware. I could possibly get back to the old firmware, if Creative were still hosting it on their site!
So if anyone happens to know where I can get hold of the old firmware, please let me know. Otherwise, I'm waiting for Creative's tech support to get back to me.