Despite Apple's warning not to try this, I successfully upgraded my iMac from OS/X 10.4.11 to 10.6! No problems, no requests to insert the original OS/X disc, nothing. I made a backup of my files before I started, but it was not necessary. All the files, and configuration information like which programs to launch on startup, position of the dock, and so on, were preserved.
The upgrade couldn't be easier. With the computer running, insert the Snow Leopard disc. In a few moments, click "upgrade" (rather than "restore"). There are a few dialog boxes presented in the first 5 minutes of the upgrade process, but after that it's entirely automatic. I went to bed so I can't say how long the process took. In the morning, the process was over and the machine was running OS/X 10.6.
Well done, Apple!