Inform the user why? What are they going to do differently as a result? I use Dropbox precisely so I don't have to concern myself with server downtime, database upgrades, etc. and I'm guessing most other users are the same way. Just sync it when the service is back up, I'm okay with not knowing.
This morning, my dad made a few changes to his files in dropbox at home, as usual, turned off the system and went to office. After a couple of hours, he calls me and asks me why his files weren't synced as they were everyday. Our initial thought was that he turned the home computer off before dropbox could sync, but as you might have guessed, it is because dropbox was down.
Not sure about everyone, but my dad could've definitely used some warning that dropbox would be down.
I'd say only notify the user if they try to push changes during planned downtime (maybe it does so already - I haven't noticed). That, or something unobtrusive, like an icon change 1hr - 30mins ahead of time. I think an actual notice would be overkill, though.
<DropboxOps> We're currently upgrading our database server. We'll require about 30-45 minutes of maintenance until everything's back up.