I posted out on Codeplex that I was having some issues with the placement of my MediaPicker field in a custom module I was building. Some suggested that perhaps my Placement naming wasn't quite right, but no way, I've placed MediaPicker fields plenty of times and never had a single issue. What was strange was that if I put the placement in my Theme's Placement file, the field would position itself correctly. However, if I cut/pasted the same line into my module's Placement file, the placement would be lost and the field would be tossed at the end of the content zone as expected.
So what was the problem?
I scoured Codeplex for answers. Right when I thought that maybe I was on my own, I came across a seemingly unrelated discussion where Sebastien posted a tip related to MediaPicker fields. It had to do with my module's dependencies. Because I had added dependencies to other custom modules in my site, the Placement of MediaPicker was being commanded by the placement files in those dependencies. By adding a dependency on Orchard.Fields to my module, my module regained control of field placement without adversely affecting the other dependent module placements.