I have been wishing there was a JPEG equivalent with an alpha channel for like forever. That allows better compositing to arbitrary background images or patterns. Now the question is how long before browsers might support it natively.
It's also supported natively on all newish (4.0+) Android devices and Chrome.
JPEG 2000 works in Safari. JPEG XR in IE. A gracefully-degrading JPEG XT is in the works: http://www.jpeg.org/jpegxt/index.html
Lossy PNG with alpha works in all browsers: http://pngquant.org (not equivalent in compression ratios, but still 4 times better than normal PNG).