Industry group including Broadcom, Texas Instruments, propose 540 Mbps 802.11n approach: The Worldwide Spectrum Efficiency (WWiSE) proposal has backing from Broadcom, Conexant (Intersil's Wi-Fi portfolio), Airgo (MIMO developer), Bermai (early 802.11a developer), and STMicro. The proposal combines some ideas first discussed in early July so that it has the highest possible throughput and the maximum possible compatibility through fallback.
An array of four receive and four transmit antennas in a MIMO configuration (4 x 4) would use 40 MHz of bandwidth, or about twice that used in current 802.11b and g, to achieve speeds up to 540 Mbps (raw throughput). But in countries in which those configurations weren't allowed, the devices would fall back to 2 x 2 antennas and 20 MHz of spectrum. This would also allow backwards compatibility with 802.11b and g. The article says that in the 2 x 2 by 20 MHz configuration, speed would top out at 135 Mbps.
The firms in this proposal are offering RAND (reasonable and non-discriminatory) licensing terms, and will charge no fees for patents in their portfolio. Tony Smith offers more insight into the patent issue and the overall story over at The Register.