The UK carrier Orange has the Lumia 900 listed on the help section of their website. Not only is the Windows Phone detailed with photos, tips, helpful tutorials and more, but the user guide is available. Now, we're not ones to jump the gun but this appears to be a pretty definitive 'yes' to the Lumia 900 coming to Orange.
The next question is "when?". With the delays that are occurring throughout the country and beyond, it could be quite the wait. Then again when the retail chains all launch the smartphone later this month, carriers might not be far behind (if at all).
The Lumia 800 and 710 are both "out of stock" on the carrier's website. With this hint at the Lumia 900 joining the two Windows Phones, this could be a sign that Orange is satisfied with sales thus far. Orange is also set to launch the Lumia 610 with NFC technology later this year.
Source: Orange; thanks Simon for the tip!