The new Transparency Support in Bridge 9.1 is a bad idea, it's impossible to view type on the transparency portion of the file, and the grid makes it more difficult to quickly view the preview. There should at least be a way to turn it off.
You can turn off the checkerboard, both in Preferences> Thumbnail, and on the bar above under the checkerboard icon, so you can switch on the fly. What you can't currently do is view it on white, switch with a right click to choose a color for the thumbnail surround, something that would let us have more control than the slider, but in the Interface preferences, you can choose the shade of gray you prefer.
Transparency support is an excellent idea for those of us who us a lot of transparent files. It simply doesn't go far enough because it doesn't yet support all files that can have transparency, nor does it let us choose the best thumbnail background color, preferably on the fly, to match what we're looking at.