To charge a iPhone4 the battery requires approx 5.3 Whrs ( Built-in rechargeable Li-ion battery 3.7 V at 1420 mAh ) with a charge efficiency of say 90% and a DC-DC converter efficiency of approx 70% you are going to need around 7-8 Whrs of AA cells i.e around 4 Hi capacity NiMh AAs or around 6-8 Alkaline AAs (for the same charge time). Neither are lightweight or small compared to the dedicated Lithium Ion rechargeable smart phone rechargers.

http://www.amazon.com/Tekkeon-TekCharge-Mobile-Battery-Charger/dp/B0014KLX9C

Perhaps the Tekkeon TekCharge with 4 AAs would be a better bet to get some charge into todays smart phones. Typical weight with 4 NiMhs will be around 280 grams (to give around 1 iPhone charge) compared to the New Trent iDual-Port Pack IMP50D 5000mAh External Battery Pack at around 155 gms (to give 2 charges). Using Lithium AA Primaries would give better performance (lighter weight) but would prove to be extremely expensive in long term use.