Unfortunately, I'm still dealing with this Droid to Droid issue. My wife and I both removed our cases (they are Seidios with the cutout on the back) and there was no improvement.
Verizon's second-level support suggested that some towers are having difficulty with Droids communicating with each other. I'm not a tech and don't know what specifics would make this so, but the rep. stated that I wasn't the only one reporting this issue.
It's frustrating because everyone else I call sounds crystal clear and vice versa, and that's true whether it's a landline or cell. One would assume that a conversation with the exact same phone model would be perfect, but mine is passable at best, and occasionally downright terrible.
I hoped that 2.1 would fix this issue, but it has not. The initial advice about the noise-cancelling mic made a lot of sense, but my wife and I have made sure we aren't covering this and still notice muffled voices when calling each other. If this keeps up, she's volunteered to swap her Droid with any other model of phone that allows us to have a clear conversation.