I've gotten a solid 11 to 12KB/s on my 28.8Kb dial-up for minutes with longer downloads. I think that 28.8Kb is supposed to be an average, and that it can go faster. Transfers may go faster because of zero/extremely minimal traffic on a server.
Downloads can probably be that high when you're downloading text like documents. Its due to compression that normaly runs over the line which can speed up the download of uncompressed files.