What I found is that the Certification "Out of the Box" (OOB) settings for Draft 2.0 11n devices operating in the 2.4 GHz band include having 20 MHz mode enabled. This was confirmed in a call today with the Alliances Technical Director Greg Ennis, who said that there is still ongoing debate within the IEEE on the implementation details of 2.4 GHz 20 / 40 MHz channel mode. So in the interest of getting the Certification process moving, the Alliance decided not to include any 40 MHz channel testing in the 2.4 GHz band.
Note that manufacturers are free to include the option of allowing the user to enable 40 MHz channel mode for the 2.4 GHz band, without violating either IEEE Draft 2.0 or the Alliance's Draft 2.0 Certification. But what happens when the product is in that mode is up to whatever the manufacturer chooses to do. So expect experimentation and a variety of "good" and "bad neighbor" behaviors from now until 11n is finally approved.
And while the Alliance does not plan to update its Draft 11n Certification process until the IEEE spec is released, there is possibility for oversight via the Alliance's "Extensions Policy". This policy, which came into effect after the Atheros "Super-G" controversy, will also be applied to Draft 11n, as described in this Alliance FAQ:
Any product with a feature extension (such as an early feature of the IEEE 802.11n draft standard) that causes severe performance degradation and interoperability issues with another Wi-Fi CERTIFIED product can have its certification revoked. The Wi-Fi Alliance® has a formal process for vendors to file grievances. This policy is in place to protect user experience, but at the same time, to allow for innovation and differentiation of products from vendors.
So if you're going to make the jump to Draft 11n product and plan to enable 40 MHz channels in 2.4 GHz, realize that anything goes when it comes to cooperation neighboring wireless LANs. You might find angry neighbors banging on your door accusing you of knocking their wireless off the air. Then again, you might find your throughput severly reduced, too.
So the long road to 802.11n continues. Hang on, folks, because Wi-Fi mark or no, there still be plenty of dragons ahead. Arrrrghhh!