"Apple in its iPhone release presentation mentioned that the image sensor is a 8MP sensor with the description 3264 pixels X 2448 pixels resolution or 7.99MP. It is spot on with OVTI's 8830 spec, whereas Sony's 8MP spec is 3288 X 2472 or 8.12MP."
Update: More analyst's speculations:
Barrons: R.W. Baird’s Tristan Gerra writes that his “field research” suggests Omnivision has lost some share to Sony in the 4S, citing anecdotal evidence that the Sony part matches the phone’s specs.
“Various websites report these Sony Ericsson smartphones feature a Sony image sensor of the exact size as for the iPhone 4S (3264×2448),” he writes, adding, “Additionally, image sensor makers typically customize their sensors specifically for Apple without necessarily releasing the specs.”
But JP Morgan’s Paul Coster comes to the stock’s defense, noting that Apple describes an a larger “active array size” to the iPhone’s sensor. That, he suggests, implies the phone uses OmniVision’s “OVT8830 CIS” chip. “The iPhone 4s sensor active array was announced to be 3264 by 2448, which is an exact match for the Omnivision OVT8830 first-generation BSI image sensor,” writes Coster, “We note that Sony’s IMX105PQ 8MPx chip has a 3288 by 2472 array.”
Motley Fool writes, again relying mostly on the sensor format: "I think I've built a pretty compelling case here for the OV8830. Dual-sourcing a component this important just doesn't fit Apple's modus operandi."