The Snow Leopard install process goes faster than prior versions of the OS because files are transferred from the DVD to the hard drive first. There's a point where that process has hung for two of us at TMO. Here's what we did.
The Snow Leopard install has, if you will, four install phases. After clicking the install icon on the DVD, files are transferred to the internal drive. Then, there is a reboot. Then, files that were loaded onto the hard disk are installed. Then, there's a final reboot.
For two of us on the TMO staff, the install process hung up on just one computer at the end of the first phase. A dialog box came up* and said that we couldn't be logged out. Upon clicking Okay, the screen drops into your default desktop image but with no menu bar and no icons. You can move the cursor, but there's nothing to click. Nothing much will happen for a long time.
At this point, remembering that the Snow Leopard install can be interrupted and resumed, I went ahead and held down the power button on the MacBook Pro, and the system rebooted. I got a dialog box that said there is a system to update, and I followed the prompts. The install proceeded normally to completion. Another staff member was able to remotely open a terminal and enter:
shutdown -r now
to achieve the same thing.
In both cases, our Macs booted back up, recognized that an install was in progress and continued without incident.
Because this exact problem happened to two of us at the same point in the install, and I haven't heard of any other issues, it appears to be a bug that can be dealt with by a reboot. If you have other symptoms or the install goes wrong in a different way, there's no guarantee that a brute force reboot will save the day. In that case, you may have to restore from backup and start over.
* No screen shots available.