I created a document in Word 2007 Beta a few months ago; I opened it just now in Word 2007 RTM, and I'm greeted with:
"This file was created in a pre-release version of Word 2007 and cannot be opened in this version."

Now what do I do?  My file is gone?

As usual, Google provides the answer: http://support.microsoft.com/kb/926054

I'm out of luck.  I have to install Beta 2 Technical Refresh, and/or my old beta 2 build:
These methods will not enable you to open an Open XML file that was saved in the Beta 2 version of Word 2007. The only workaround is to open the file in the original release version of Word 2007 and then resave the file. When you do this, the file will be saved in the newest file format. The recipient of the document can then use an earlier version of Word with the Compatibility Pack to open the file.

Yes, I clicked the link, and told Microsoft that the information was completely useless.