We do something like this in our app, where we provider our users a
link to open Office document files in Google Docs. Unfortunately, this
means that each time a user clicks this link, we're uploading the doc
to Google Docs, which creates a new copy of it in their docs list.
Ideally we'd like to be able to upload the doc as an "unsaved draft",
so that it wouldn't exist in their docs list until/unless they saved
it. This feature is actually available in GMail, where you can
open .doc, .xls, and .ppt attachments in Google Docs, and they're
opened in this draft mode. It would be great if that capability were
exposed in the Documents List API. I submitted an enhancement request
for it, if you're interested in seeing this capability added, star it
and maybe it will get some attention:
http://code.google.com/p/gdata-issues/issues/detail?id=1412