> jpegtran can do some other things losslessly as well - flipping, cropping, rotating, transposing, converting to greyscale

Ok so those are only lossless in specific situations when the image dimensions are a multiple of the dct block size. And of course grayscale and cropping are lossy by definition. A better way to say it is that it can perform some transformations without recomputing and compressing the dct although if you pass the -optimize option you are still redoing the Huffman tables.