When you go into Manage Apps, you can hit the Menu key and choose the Filter option. Choose the "All" button and Quickoffice will be in the list. I just confirm this on my Droid.
I too have been having the same problem and when I filter "all" and select Quickoffice there are no defaults set. The "clear default" button is grayed out? Any thoughts
Boy do I feel dumb, I figured out my problem. When I bought the full version of Docs to GO, I deleted the trial. Didn't realize the what was downloaded was only a "KEY" and still needed the trial installed. What was bought was simply a key to unlock the trial version not a "full install" like I had originally thought.