I have the Acrobat X Pro that came with CS5.5 Design Premium (Mac) and am unable to export to Word because there is no such option listed under File->Save As. Oddly the free demo version I have on my Windows 7 laptop has the option.
I've tried reinstalling to no avail.
This is especially irksome since Adobe's home page is currently highlighting this nonexistent (for me) feature.
I've seen this behavior myself from time to time. The File > Save As > Microsoft Word > Word Document option just disappears. Try opening a second PDF file, then go back to your first one. The option should be there.