InCase Snap Battery Case 60 is as close to "just an add-on battery" as a case can be. Barebones, bulk reduction at its best. Added battery life is not great, but still much better than just the internal iPhone battery. The case is stylish and modern, with a minimalistic look, that wlll fit your iPhone nicely.
"Taking obvious pride in its minimalist approaches to both iPhone case and battery development, Incase has now gone one step further with the Snap Battery Case for iPhone 4 ($60), a deliberate effort to reduce the hybrid of spare power cell and protective shell down to its simplest possible implementation."
Read the full article at: http://www.ilounge.com/index.php/reviews/entry/incase-snap-battery-case-for-iphone-4/