This is always something that's interested me. I've sat at both sides of the table. I've been the developer who has had a first look at designs after they've been sold to the business, only to have to let the designer know almost none of it is actually technically possible in the timescales.
Similarly, I've been the UX designer who has been on the receiving end of a far too development heavy team and been told to work in boxes with no real movement. I know it's good to know a bit of each discipline, but it's very rare for 1 person to completely own both sides in a single project.
What level of relationship should UXers and developers have? Should developers be engaged at proposition / concept stages, or does this hamper and dull creativity?
Should designers be free to run with a design with no creative restraints so there is always innovation through design?
In your experiences, what has worked well, and what would you make sure you never done again?