I am also experiencing problems connecting my Xbox 360 S to my wireless gateway (Arris TG862G/CT). I have no problems connecting my older Xbox 360 with external wireless network adapter. I saw several recommendations of EMTAs. What brands would you recommend?
If you're looking for a simple fix to get your xbox s w/ kinects to connect to you "lovely" Arris Tg862 modem supplied by Comcast/Xfinity wirelessly, simply change your security mode to "wpa2-psk (AES)" from the "WPA2-PSK (TKIP/AES) recommended.
In the case of the Arris TG862, you can even use two phones if you subscribe to two lines. A common practice is to use one line for voice and the other for fax.