NFS Carbon Widescreen Fix
It's a plugin, which allows the game to run in widescreen resolution, defined in nfsc_res.ini, without running any external tools. Exe v1.4 (7 217 152 bytes) is supported. To install, copy content of NFSC_widescreen_fix.zip in NFS Carbon root directory. By default, if the X and Y parameters in nfsu_res.ini equals to 0, the plugin will run the game in your desktop resolution.
More info and feedback at gtaforums topic.
This game, like Most Wanted before it, does not support any resolution above 1024x768 without hassle. The easiest way to get Widescreen, or any other "uncommon" (EA's words) resolution - like, oh, 1280x1024 - is to use Racer_s's wonderful NFSCRes
Other methods include hacking the program via ArtMoney while it's running, or using a UniWS to create a NFS:Underground executable and renaming it appropriately. These all accomplish the same thing, but require much more effort.
The game was doing poorly but acceptably until now. But there's one last thing to add: performance chugs horribly above 1024x768. My 256MB GeForce Go6800 breezes through Most Wanted at 1440x900 at the same settings without skipping a beat; in Carbon, even at 1280x720, it rarely if ever crests 30 fps. I have also tried it on a 256MB GeForce 7900GT at 1280x800 and 1280x1024, which gets, somehow, even more abysmal framerates. This is inexcusable for a game who's predecessor (which is based on the same engine) ran just fine at Widescreen resolutions (albeit also Vert-). For shame, EA, for shame.