[quote]Will your injection device adhere to 802.11 contention?[/quote]
I think it's about the driver that controls the device rather than the device itself. Actually, "I think" was an understatement:-) I'm sure of that. Now, whether it adheres to 802.11 contention is something you need to ask TamoSoft, as they make the drivers for the Wi-Fi adapters used for packet injection. I have no idea.
If you look at another page in the their manual, http://www.tamos.com/htmlhelp/commwifi/nodereassociation.htm, you will see that they claim that one can easily send deauth frames with their software to initiate EAPOL key exchange, which is needed for WPA-PSK decryption. Pretty close to the task we're discussing here. Since that works (and that's something I'm sure of, as I tested this myself), either it adheres to 802.11 contention, or 802.11 contention is not an issue in an environment with light WLAN utilization, especially if you send multiple packets in a row (you just get lucky most of the times).