Well I have to think that Tolkien knew what the OE word meant so whatever those were-worms were in the Hobbit novel they werent the massive earth movers we see in the film. Tolkien wouldnt misuse a word like that.
The bottom line is that PJ butchered this film and I hope he's never allowed to touch another Tolkien writing again. Even though the LotR trilogy he did was off base in so many ways, I was willing to give him a pass on that one because it seemed like he liked and, even more importantly, respected the work.
As far as were-worms go, there are, in my opinion, two possibilities. As they were mentioned only once, and by Bilbo, it's likely that they didn't even exist, but were part of hobbit folklore (and, yes, I realize Bilbo never really existed either

). The other possibility is that, they did, in fact, exist (or existed at some point in the past). "IF" they existed, they were almost certainly some sort of dragon (non-flying) in the mold of Glaurung (killed by Turambar). Glaurung was often referred to as a "Worm". There is no indication whatsoever of earth delving/moving worms doing this sort of work for Orcs. None. In fact, both Orcs and Dwarves are known to be tunnelers and built underground strongholds. Had Professor Tolkien decided that Orcs used this sort of creature, I'm sure we would all have heard about it.
This was yet another thing that was just made up and added nothing to the story. PJ probably read "Dune" and thought "Wow, I have a great IDEA"
EDIT: In PJ's world, those worms would have joined the battle and fought on the side of Sauron (unless they were like certain trolls and couldn't live in the sunlight and if that were the case, they would have attacked after sundown). It was just a worthless attempt at action or something along those lines.