The ceramic needs to be built out. In the following pic you'll see on either side of the window, where the accent stripe comes into the outlets and switches. That stripe, without doing anything to the tumbled marble, would've stood proud by about a shy 1/8", in which case, as you're finding out now, there'd be a gap behind the face plate. You might want to pull the ceramic pieces and build them out a bit.

Bill, is there an easy way to pull tile out, once it is set in thinset? Prior to using the geckos (ha!ha!) to cover some of my troubles, I had a crazy idea of simply prying a tile out, and managed to chip it AND its neighbour in the process.....

What if you scribed a line on the switch plate cover where the two different thickness are and removed some of it (the switchplace cover) with a dremmel tool so that it sits flush?

You can't be prying on the tiles. It just won't happen, especially being set with thinset. You're going to have to break them out with a hammer. I would not tap on them any harder than necessary to remove them, or you might end up destroying the wallboard in the process.

Oooohhh...I like that backsplash! I've never seen colored grout enhance a design so well. That looks awesome!

Thanks for the picture Bill and for the replies....I went ahead and grouted it today. It actually looks real nice and isn't as bad as I thought it would be, especially with this being my 1st tiling attempt.