I swapped out the factory battery for a 1600 mAh Seidio. I know that it fits snugger inside the phone but it REALLY feels like the previously-loose battery door is snug, too. Anyone else experience this?

No, but the loose door is one of the first things I've noticed about my 8900. Thanks for the idea of something to look into though.

I found a couple pieces of masking tape on my battery puts enough pressure on the battery door to stop it from sliding around.

In the time since I initially posted, I swapped the batteries back temporarily. That did the trick. It's still a tad loose, but nowhere near what it was. The battery life is noticeably better, too. Not 15%, I don't think, but better.