Setting background color/pattern of Inkscape workspace to distinguish transparency

In InkScape 0.48:

File > Document Properties > Page > General > Background

Move R, G, and B to 0. Adjust A (Alpha) to your preference (experiment to set to your liking).

Note that this approach will set the color you choose as the background for the saved/exported image.

In InkScape 0.92:

File > Document Properties > Page > General > Background > Checkerboard background

Since version 0.92 (January 2017) you can instead choose a checkered background. This won’t effect the background of the saved image, but will override the background shown in InkScape.