It seems pretty clear that Leopard is a buggy release. I've updated two machines of our five, and had problems with both installs.
On the first, the problems were fairly minor - mainly lost preferences.
On the second, Leopard effectively bricked my Powerbook, making it impossible to log in or use the command line to try and fix the problem. I've set up a trip to the Apple Store this evening - hopefully they can fix the issue.
It seems like the bugginess of this update brings out the worst in Apple fans. Most of the feedback I've received is that the problem is my fault and I should have known better.