I got the following error when I tried to install Office 2007 Professional this morning:

The 2007 Microsoft Office system does not support upgrading from a prerelease version of the 2007 Microsoft Office system. You must first uninstall any prerelease versions of the 2007 Microsoft Office system products and associated technologies.

Correct the issue(s) listed above and re-run setup.

I had uninstalled all of the software except for Office 2003 Web Components which as it turns out needs to be uninstalled.  I hoped for the best, but I still got the same error.  In order to resolve it I had to dig into the temporary log file that the setup application creates in the temporary directory (C:\Documents and Settings\[Login Name]\Local Settings\Temp.  The file was called something like SetupExe(2006....).log.  Near the end of the file was the following text:

Catalyst beta product conflict check failed. Office Beta product conflict is detected on the computer, productcode={30120000-001F-0409-0000-0000000FF1CE}
Showing message Title: 'Setup Errors', Message: 'Setup is unable to proceed due to the following error(s):
The 2007 Microsoft Office system does not support upgrading from a prerelease version of the 2007 Microsoft Office system. You must first uninstall any prerelease versions of the 2007 Microsoft Office system products and associated technologies.
Correct the issue(s) listed above and re-run setup.
'
Message returned: 1
Error: Catalyst intermediate PreReqCheck has failed Type: PreReqCheckFailure.
Error: Catalyst intermediate PreReqCheck has failed Type: PreReqCheckFailure.
Error: Catalyst intermediate PreReqCheck has failed Type: PreReqCheckFailure.
Catalyst execution finished: 11/15/2006 10:26:19.  Return code: 30066.  Exception caught: PreReqCheckFailure.
PERF: TickCount=2456437 Name=RunSetup Description=End function

So, off to the Registry I went to delete every instance I could find for the listed product code.  After clearing them out I restarted the setup application and everything worked fine.  Hopefully, this information will be useful to someone else, because I couldn't find anything on the Internet about it.