Good news for people with 2 or more Xbox 360's hooked up to the same router, there is another router to add to the small list of routers that will allow 2 or more Xbox 360's to have Open NAT!

I purchased the D-Link DSL-2740B (http://www.amazon.co.uk/gp/product/B000FL8NXK?ie=UTF8&tag=xbo360for-21&linkCode=as2&camp=1634&creative=19450&creativeASIN=B000FL8NXK) (ADSL Customers) today without any reviews saying it will or will not work (I had a should work on The Unofficial Guide to Xbox Live), so I took the gamble and it worked :D 2 Xbox 360's both with open NAT. No tweaking or port forwarding was required! Just setup and away you go :)

Here is a list of other routers that should work courtesy of the Unofficial Guide to Xbox Live (http://www.unofficialguidetolive.co.uk/):

Cable Users

Belkin (http://www.amazon.co.uk/gp/product/B0006374LE?ie=UTF8&tag=xbo360for-21&linkCode=as2&camp=1634&creative=19450&creativeASIN=B0006374LE)F5D7231uk4 (http://www.amazon.co.uk/gp/product/B0006374LE?ie=UTF8&tag=xbo360for-21&linkCode=as2&camp=1634&creative=19450&creativeASIN=B0006374LE) (version 2) (firmware F5D7231-4_UK_5.01.03) - Ensure you enable UPnP.
Dlink DIR-655 Extreme N Gigabit router (http://www.amazon.co.uk/gp/product/B000PLSAII?ie=UTF8&tag=xbo360for-21&linkCode=as2&camp=1634&creative=19450&creativeASIN=B000PLSAII) (tested with firmware 1.03, higher versions of firmware may also work)
Dlink DIR-300/301 Wireless G Router(Also known as DKT-110 when bundled with wireless PC adapter
Dlink DIR-825 (http://www.amazon.co.uk/gp/product/B0027W3B46?ie=UTF8&tag=xbo360for-21&linkCode=as2&camp=1634&creative=19450&creativeASIN=B0027W3B46) (firmware 2.01EU, higher versions may also work)

ADSL Users

D-link DSL-2640B (http://www.amazon.co.uk/gp/product/B000WW75E0?ie=UTF8&tag=xbo360for-21&linkCode=as2&camp=1634&creative=19450&creativeASIN=B000WW75E0) also known as DKT-710 (firmware 3.06, EU_4.00 - higher versions may also work)
D-link DSL-2640R (http://www.amazon.co.uk/gp/product/B0027DO6XK?ie=UTF8&tag=xbo360for-21&linkCode=as2&camp=1634&creative=19450&creativeASIN=B0027DO6XK) - which replaces the DSL-2640B (firmware UK_1.06 - higher versions may also work)
Belkin F5D7633uk4A (http://www.amazon.co.uk/gp/product/B0006Z8JZW?ie=UTF8&tag=xbo360for-21&linkCode=as2&camp=1634&creative=19450&creativeASIN=B0006Z8JZW) (firmware 1.00.17) - Ensure you enable UPnP, details for Belkin here (http://www.unofficialguidetolive.co.uk/XBoxLiveTheGuide3.php?title=My%20NAT%20Setting%20i s%20Strict/Moderate%20what%20can%20I%20do)
Belkin F5D9630-4 (http://www.amazon.co.uk/gp/product/B002XEWE96?ie=UTF8&tag=xbo360for-21&linkCode=as2&camp=1634&creative=19450&creativeASIN=B002XEWE96) (firmware 1.00.04) - Ensure you enable UPnP.
BT HomeHub 1.0-1.5 (http://www.amazon.co.uk/gp/product/B001EOTIKE?ie=UTF8&tag=xbo360for-21&linkCode=as2&camp=1634&creative=19450&creativeASIN=B001EOTIKE) (firmware 6.2.6.E onwards) - BT ISP's only
BT HomeHub 2.0 (http://www.amazon.co.uk/gp/product/B001JT0HYK?ie=UTF8&tag=xbo360for-21&linkCode=as2&camp=1634&creative=19450&creativeASIN=B001JT0HYK) Wireless N version (firmware 8.1.A.E (Type A) onwards)
Zyxel 600 HW T1 (http://www.amazon.co.uk/gp/product/B0015RY0EY?ie=UTF8&tag=xbo360for-21&linkCode=as2&camp=1634&creative=19450&creativeASIN=B0015RY0EY)
Sitecom WL-174 (Firmware 2.9.5.0 - 3.5.18.9)