Back in 1989 a very clever, and funny, man called Douglas Adams got very frustrated because it was difficult to keep documents in sync across multiple machines without jumping through hoops and even manage content from different sources within a document elegantly.
Luckily in the years since Office file formats (both Office Open XML and OpenDocument) and the various Office suites have become smarter and more flexible (and dare I say "open") but the problems of keeping things in sync so the user isn't concerned where he edited the document when he wants to print it are not quite solved just yet.
Things like Groove and FolderShare are making it easier, and hopefully the promises of computing in the cloud will continue to accelerate the pace of development.
Sadly Douglas is no longer with us, but memories of Frank the Vandal live on...