Cisco has officially entered the Transmit Beamforming (TxBF) game. Let's take a look at their thankfully-updated (inside joke for those of you who missed the first version) whitepaper.
Cisco's TxBF implementation, called ClientLink, is the closest that I've seen in the market thus far to standards-based TxBF, though it's not really all that close to the 802.11n draft implementation. ClientLink doesn't use explicit or implicit feedback mechanisms (because those are 802.11n draft features), but instead uses the 802.11a/g client's uplink traffic as the feedback mechanism - even though their whitepaper explicitly states, "without requiring feedback." Using uplink traffic as the feedback mechanism has the obvious problem of sub-optimal beamforming if there isn't a continuous stream of traffic between the AP and client, but there isn't any other way of doing it really. One thing to point out here is that the smaller the beamformed area, the more continuous uplink feedback is needed.