Try this:
int width = bm.getWidth();
int height = bm.getHeight();
int[] pixels = new int[width * height];
int curPixel;
for (int columnNum = 0; columnNum < width; columnNum++) {
for (int rowNum = 0; rowNum < height; rowNum++) {
curPixel = pixels[rowNum * width + columnNum];
// do stuff with curPixel
}
}