How can I change Inkscape's default export background color from yellow to white?

At least for documents created directly in Inkscape, File > Export Bitmap... works as expected. If the document has transparent background, the exported PNG becomes transparent. If it was white, then PNG becomes white.

Please check File > Document Properties > Page > Background and check the aplpha channel (A). On RGB tab set all values to 255, including A.

Checked using Inkscape 0.47 on Ubuntu Linux 10.04.

If your problem happens only on convertion of SVG files created elswhere (not in Inkscape), then you need to post that file, so we can try it out.