Kevan points out (by email) that with your 3G handset acting as a Conversational TiVo there'd be a lot of uncomfortable "Is that thing on?" going on. Okay, so: you're in a group of people, all with your handsets out. The handsets talk via Bluetooth to register interest in a conversation. A group is determined by looking at the various phone addressbooks, and comparing volumes of components of the conversation. To listen to something that happened on your own recording, you need approval from other members of the group.
Or another solution. A gizmo strapped to your neck over your voicebox has three modes of vibration which subtly alter your voice: one mode for public, one for private, one for this group only. You can alter which mode with a simple switch. The recording device extracts and understands this vibration mode before allowing playback and uses it for Digital Rights Management, ie a device will refuse to play speech that has a "private" voice watermark imprinted.
There are more 3G applications in the Interconnected archives.