Historically speaking, new iPhones always used existing iPad architecture, down clocked to save battery life. The only special case was RAM in iPhone 4, which had more RAM than existing iPad.
1st generation iPad (April 2010): 1 GHz A4 (single core), 256 MB RAM, PowerVR SGX535 (single core)
iPhone 4 (June 2010): 800 MHz A4 (single core), 512 MB RAM, PowerVR SGX535 (single core)
2nd generation iPad (March 2011): 1 GHz A5 (dual core), 512 MB RAM, PowerVR SGX543MP2 (dual core)
iPhone 4S (October 2011): 800 MHz A5 (dual core), 512 MB RAM, PowerVR SGX543MP2 (dual core)
3rd generation iPad (March 2012): 1 GHz A5x (dual core), 1 GB RAM, PowerVR SGX543MP4 (quad core)
I suspect 6th generation iPhone will use 800 MHz A5X (dual core) with 1 GB RAM and Power VR SGX543MP4 (quad core).