I did spend some time looking at battery packs at the local stores but some wont get you a full charge.  The Kensington looks decent but I think its over priced and a bit too bulky for me.

I ended up with the Belkin micro auto charger as part of my travel pack (when not in car charge the phone using the wall charger or computer via usb sync cable).  Figured its less to carry and simpler
http://www.youtube.com/watch?v=K9T9kOyrhcU

There is a few battery pack out there. Mophie,Exogear and Mili are more to the reputable side. My personal recommendation, go for exolife from exogear. They are the silmmest out of those 3, looks good and the audio jack hole fits with any earphones unlike the rest. The battery pack is capable to recharge ur iphone to 90% from 2%. Macstudio is selling it for RM318 but they are out of stock at the moment.

Check out http://www.exogear.com/products-iphone-4.html for more info.

I am letting go of mine, do PM if you are interested.

Get the Sanyo Eneloop Mobile Booster,
power rated 5000mAh, quite powerful,
but charging abit slow, coz it charge at 500mAh,
so, would take about 3-4 hours for fully charge compare you charge with the normal power adaptor.

Price wise <RM200, quite worth it,
but the way, do look carefully, there is 3 model in total,
get the 5000mAh model, price wise not expensive a lot compare to low end model.
There is one forumer selling, can search in trade zone.