Voice chat was one of several upcoming features touted by OnLive during its panel for potential publishing partners at GDC. Its rollout wasn't dated at the time -- merely "by the end of the year" -- but it turns out there wasn't much of a wait at all; it's available now.
By enabling the voice chat beta option in the OnLive client, users of the service can access two chat channels: one for in-game yacking (open mic), the other for talking with other players while spectating in the Arena (push to talk). Corded and USB microphones are supported on PC and Mac; the OnLive MicroConsole adds support for Bluetooth headsets and VR goggles.
Okay, we were kidding about the goggles -- but you have to admit it'd be pretty cool.