Why I am having the problem? -- Hmm, that's my question!
I explained this behaviour with this very simple example. I'm having this problem in a complex poster and asked me, why some text boxex were hidden in the PDF file and others not. So I reduced the complexity of the poster and only after removing the imported graphics the text boxes appeared in the PDF.
Problem seems to occur more often: http://forums.adobe.com/thread/838524
And I am having it too on Adobe Standard 9.4.4.
Text in text box from Word 2010 is not output in PDF (even not hidden). The text box background is copied over in the PDF. Reproducible. And if I put multiple text boxes on each other, sometimes a few or parts of characters appear.
I find Microsoft Save as PDF not an option, it has numerous worse bugs. Good to know that the problems also appears with Acrobat X. That save me an upgrade to test whether it has been solved by a newer release.
I gave up and wanted to go back to the scenario 'Just patch it up in Adobe Acrobat'. After changing the background color fill back from green to 'No color' in Microsoft Word, I printed it once more to PDF.
And voila, problem solved!
I tried the procedure of giving the text box a non-transparant color, printing it once, removing non-transparant color and printing again on another textbox. Again, the problem for that text box was solved too. Now the textboxes all have their text appearing in the PDF output. Lucky me.
Microsoft has released a hotfix for the problem like the one mentioned by you. Please install the hotfix and let others know whether the problem is solved or not.
The URL of the hotfix is http://support.microsoft.com/kb/2596551.