I've spoken with a field engineer about building something similar to PlanGrid, but their concerns revolved around the ability to convert to AutoCAD automatically. From what I gathered, generally on a site people will draw everything out on paper, and then take the plans back where people will draft them up in AutoCAD. Unfortunately, CAD isn't an open source format. If these guys can make that happen, I think this would be HUGE.

If anyone from PlanGrid reads this, here's another idea that makes this useful (if you haven't already done this) -- for site checks, right now field engineers just have to go around and check things off to confirm they are where they supposed to be on the plan. You can use the camera on the iPad to snap a shot and attach it to the coordinates in your plan to actually guarantee it's there.