Hi, you may already know about this but just giving you a heads up in case you don't.
Microsoft are changing Office Live documents in SkyDrive to resemble the Metro interface, they have also started offering you the option to opt in to a preview of the new Office Web Apps. I currently embed spreadsheets into my WordPress site.
I decided to opt in and then found that any new items I went to embed simply didn't appear. I noticed that the url Microsoft provide you with for the preview has changed and no longer starts http://r.office.microsoft.com, but starts skydrive.live.com and I think WordPress doesn't know yet how to interpret it.
Giving you the heads up in case this becomes a wholesale change and all of a sudden WordPress user's can no longer embed documents/spread sheets etc.