I've just started playing a heavily modded game of Skyrim with Hearthfire and Dawnguard. I have the Steam versions. I'm a little worried about what's going to happen next week when Dragonborn hits. I have Steam set to not update - but I've burned by that before. I have not pre-ordered Dragonborn - I'm going to get it but not until I'm done with my current game.

Are we expecting a big patch? If Steam "updates", would it help If I back up everything and then try to copy it back in at a later time? How badly did the arrival of the previous DLC hose your ongoing games?

Dragonborn only requires the 1.8 patch, which all PC users have already.

There will be no patch when Dragonborn relases, and thus, no mods will become broken.

Really? That's the best news I've heard today .... hopefully won't be the last good news I hear, though.

There's no non-Steam version of the game or DLC.
Backing up your game is often sensible, and you can copy things back if it updates. So it won't hurt (unless you're low on HDD space.)
I don't think we're expecting a patch, and if you're not planning to get DB immediately, then I wouldn't worry too much. In the past, DLC have given some serious setbacks, but they were accompanied by patch, or when you actually bought the DLC, so you should be fine.