It's not as dramatic as .Wav to .Mp3, but I messed around with the Jpeg2000 file plugin for PS, and was able to save things 2 and 3 times smaller at the same quality. If they could perfect this sort of thing, broadband internet would be useless to most people. I'd like to know why IE doesn't support this yet (maybe they think it's a future threat to MSN broadband sales).....
http://www.jpeg2000info.com/
http://www.leadtools.com/SDK/Raster/...00-Example.htm
http://www.leadtools.com/sdk/raster/...n-jpeg2000.htm
http://www.dspworx.com/primer_jpeg_vs__jpeg2000.htm