One marginal example: Rumor has it that the A5 ARM chip used in the iPhone 4S integrated technology from a company named "Audience" to better handle voice recognition for Siri:
As time goes by, I'd expect "third party" SoC vendors to offer their customers an ever wider range of custom silicon, but there will likely always be advantages to being the "first party".
Reports indicate Siri computes primarily in the cloud (the whole sound file is sent), and that it's not on iPhone 4 for other reasons (e.g. reduce server load, increase 4S sales)
That is true, as people have ported Siri to the iPod Touch and iPhone 4, and it is performing exactly the same. Therefore, they either emulated a whole chip (highly unlikely) or everything is processed in the cloud/on the A4/5/#.
The A5 has several integrated processors for specific purposes, specifically relating to audio and video. The Audience chip is likely one, but there are also processors specifically for the purposes of handling video encoding or decoding, and other things, probably like cryptography.
Apple was not specific about what all the coprocessors are or what they do, which is why I'm being vague, but they have publicly announced that these are integrated into the SoC.
So the "Rumor" part is that one of them is from Audience. That there are custome co-processors has been revealed publicly.
http://www.macrumors.com/2012/02/06/audiences-earsmart-techn...
As time goes by, I'd expect "third party" SoC vendors to offer their customers an ever wider range of custom silicon, but there will likely always be advantages to being the "first party".