The revealing evidence within the iOS 4.3 firmware indicates that the A5 processor bears the codename S5L8940. Interestingly enough, a kernel file, which is speculated to be written for the iPhone 5, was also found within the 4.3 update. Browsing through the kernel file, pictured below, shows references to the exact same S5L8940 processor codename. Putting the hints together is almost concrete proof that Apple's A5 processor is what might make the iPhone 5 tick.
That does not come as that big of a surprise, actually. When it was launched last summer, the iPhone 4 had the same single-core A4 processor, which had already debuted in the iPad earlier. That has already led many to believe that the iPhone 5 may be treated in a similar fashion with an A5 processor upgrade when it gets launched later this summer.
source: iClarified via Mac Rumors