Yeah, having Siri or your equivalent voice assistant listen in during calls could be interesting. Reminds me of Jane from Children of the Mind.
There could potentially be major privacy issues, though.
I agree about the privacy issues. Siri as currently implemented requires sending data off to the Internet, which is one reason why I don't personally use it very much. One of the technical reasons that Siri itself can't do this currently is that on some providers it can't do data and voice concurrently on the GSM radio. I don't know how much of Siri could be done locally on the phone, but that can't be far off.
I would love it if Siri was paying attention to things said during the call. It could suggest calendar entries, notes, and reminders based on things said.
Then of course, all of the transcripts would be searchable via Spotlight or whatever indexing system one uses.
This reminds me that it annoys me that there are no useful actions available on iOS for working with voicemail. I often would love to forward a voicemail to email in order to delegate a task.