If you're still waiting for the WM6 upgrade for your WM5 Smartphone edition so you can use Mobile Office, this is your lucky day. Someone's taken the Office portion of the WM6 ROM, extracted it, and made it compatible with WM5 smartphones!
Just head over to the download link, grab the file, and install. You'll need the .NET Compact Framework in order for this to work (it's a free download), and in order to create a new create a new document you'll have to open up an empty one and save it as a new document. Weird, but them's the limitations.