I know they tend to cut out character development that isn't Harry, BUT - the movies started inserting bits of R/Hr buildup in the second movie already. They did more in the third movie. They may not have done the Yule Ball that well, but they did it. They are clearly building up to it, and Kloves is even coming back for movie six so he knows what they've been leading to. And plus, if you cut that whole subplot, neither Ron nor Hermione would have a thing to do all movie until the Death Eaters show up! ;) It's a Harry & Dumbledore, Voldemort, Snape and Draco story, but they need to have scenes for their other two young stars - if they weren't already there they would have had to create them, IMO, to keep Rupert and Emma on the screen.

So putting it all together, I think there's no way they're suddenly going to drop the ball on that relationship for movie six, and that means doing the R/L thing.

Plus, JKR said in the TLC/MN interview that when she was working on the Won-Won/Lav-Lav bits, she kind of pictured Rupert attending the casting for Lavender. Sounds like a command from on high to me!