Reproduced with Inkscape 0.47, 0.48.0, 0.48.1, 0.48.2 (on Mac OS X 10.5.8): all exported bitmap images report 7 unique colors in GIMP (2.6.11).
Inkscape 0.48+devel r10697, cairo 1.10.2 differs slightly: exported bitmap image reports 15 unique colors in GIMP (2.6.11).
Number of unique colors appears to vary depending on the color used in Inkscape (not reproduced e.g. with #ff0000, #00ff00 and #0000ff).
Reproduced with Inkscape 0.47, 0.48.0, 0.48.1, 0.48.2 (on Mac OS X 10.5.8):
all exported bitmap images report 7 unique colors in GIMP (2.6.11).
Inkscape 0.48+devel r10697, cairo 1.10.2 differs slightly:
exported bitmap image reports 15 unique colors in GIMP (2.6.11).
Number of unique colors appears to vary depending on the color used in Inkscape (not reproduced e.g. with #ff0000, #00ff00 and #0000ff).