Just saw that the Mophie 3G is now available for pre-order:
mophie - iPhone Battery, iPod Cases, iPhone accessories and more! - Juice Pack - iPhone 3G (pre-order)
Easily the most integrated and low-profile solution, I'm looking forward to it. $99 ($105 with shipping) now, shipping in late September.
Design looks a little better too - the speaker cutouts aren't quite so pronounced.