I finally found the cause of my intermittent issue, whereby Lion would sit sulking at the login prompt after booting up and no keyboard input or mouse click would appear to work.
They were accepted, as after a lengthy delay they’d appear in the login boxes and the boot would continue. Looking in the console for errors turned up this code: Unable to open IOHIDSystem (e00002bd) Which, while a clue to google with, isn’t much help.
There are lots of people with the same problem, but very few solutions, so I was pleased to get a hint when one of the search results mentioned an Epson connected by firewire. I have an Epson scanner, that is connected by firewire, it was left on from the last time I used it, turns out it’s the culprit.
If the scanner is on when I boot the machine then I get the problem, scanner off, problem gone. So my problem/solution appears to be related to firewire connected devices.
Hope this helps someone.