I'm using Gimp 2. The default grid-style transparent background makes it incredibly difficult to get a sense of what the image will actually look like, when I finally display it over a background. Is there a way I can make the transparent background display as something other than the default grid, for example as a simple white background?
Yes, that would be very easy, I just opened up Gimp 2 my self and tried it out to make sure! :p Anyway, in the layers panel, right click and choose New Layer.
Then make sure you select White for the Layer Fill Type.
Now, click OK and move the new white layer to the bottom of all the current layers.
To change the layer color, select it and just paint it in with any color you want!
Hope this helped.
Adding a background layer is a nice way to try different backgrounds.
The more boring way would be to set the transparency style in the display preferences, "[...] either to a different type of checkerboard, or to solid black, white, or gray":
This is also described in GIMP's docs.