These are the last two facts in my five day post Ten Facts About 802.11n, if you missed any and are considering to upgrade you may want to be sure and check out the last 4 posts.
Because of the use of MIMO and spatial multiplexing, 802.11n cells will be less predictable depending on environmental factors. Without obstructions, the traditional WLAN cell is spherical, but 802.11n may take an entirely different shape altogether.
As you begin migrating to 802.11n, you need to be sure that the devices you choose are Wi-Fi certified and compliant with the 802.11n draft 2.0. The Wi-Fi Alliance has begun certifying equipment from different vendors to ensure successful migrations. In order to ensure interoperability of access points and clients you should upgrade to devices certified by the Wi-Fi Alliance.
For more important facts about 802.11n, benefits of migration and issues you may confront, you should read the Cisco White Paper 802.11n: The Next Generation of Wireless Performance.