Robin Good takes a look at real-time sharing of Microsoft Office documents via Instacoll and Conferral.

Instacoll, fully integrates with your Office tools and provides an easy and immediate approach to PowerPoint share presentations, Excel spreadsheets, Word documents, without needing to leave those applications to launch a separate tool.

Conferral adds to the same approach the ability to send files of any size to connected participants as well as the opportunity to upload Presentations to be viewed on-demand by your invitees (anyone can login at his own time and watch the presentation online).[Robin Good]

I haven't reviewed either, but his look at both is comprehensive and the comparitive information is quite useful. I would be more tempted to use Conferral, since it does not require the second user to have any of the shared programs to collaborate; however, Instacoll does have a good price advantage and offers a free version for one-to-one collaboration.

Many situations come to mind where this could be useful. Particularly cases where you are sending back and forth via email edited files with trackbacks. Co-editing would remove all this hassle. Now, if only this were more seamlessly integrated into an IM...