As a film project, The Hobbit will be a prequel to LOTR (released after, but taking place before). However, it was not written that way--Tolkien didn't follow up LOTR with the Hobbit as a way to go back and explain the back story he left out. Other than the discovery of the Ring (and Sting and Glamdring), there is very little that connect the story of The Hobbit to the "trilogy." However, the film audience will be watching The Hobbit with information about future events that wasn't available to readers of the original book, because Tolkien hadn't invented it yet. (There are even discrepancies like talking wargs.)
I think they are going to try and come up with as much material as possible to make this a functional prequel that provides backstory and leads up and ties in to LOTR, because without that it's pretty much an entirely separate adventure that takes place in an (almost) entirely different part of Middle Earth. I am expecting a certain level of "Oh yeah, Anakin made C-3PO" moments forced into the project just to connect it to LOTR in ways that weren't originally intended--because it simply wasn't written yet. Pretty much the only "important" thing that happens is that Bilbo finds the One Ring--in terms of setting up LOTR.
People unfamiliar with the story are going to want to see more events setting up the trilogy, and that's why they are going to pull concurrent events from the appendices--to pad the two films and link them to the original trilogy. I think they will do it far more successfully and with greater restraint than Lucas showed in the prequels, but I still expect them to link The Hobbit to LOTR in a lot more ways than you find in the pages of the book itself.
I think we need to set up a sticky thread saying WHY SARUMAN, GALADRIEL ETC ARE GOING TO BE IN THE HOBBIT so we don't get the question every other page!
Thanks for the nice reply though! 
