Read this section in the UDN for the UDK
Widescreen
We design our games for one FOV (16:9) so it's going to look differently on the other (4:3). Having fixed horizontal FOV seems like a good choice for games that don't have a lot of vertical, so 4:3 is still playable and you don't get tunnel vision. It means that at 4:3 you will see more compared to widescreen, but we haven't seen that as an issue if the game is designed for 16:9.
If you design your game for 16:9 locked vertical FOV, you don't get a wider FOV for 16:9 but a narrower for 4:3. Properly balanced this means you get tunnel vision on 4:3. If you don't want that, having a wider than normal FOV with 16:9 means that you not only render more pixels, but also most likely more triangles/ geometry for mostly horizontal levels, causing perf to take an extra hit. From our design standpoint, locked vertical FOV is not a good approach if your main focus is 16:9.
We've shipped with locked vertical FOV for past PC games as most folks had 4:3 screens and it also made first person weapons easier, but focusing on 16:9 means fixed horizontal is preferable, which is what we are using nowadays.
I officially hate Epic right now :x
Edit: Honestly, this method makes sense and works perfectly fine
if Eyefinity/TH2G/Nvidia Surround goes Hor+ and the 16:9 FOV is not extremely zoomed in to begin with, however most ports have super zoomed in FOV's to begin with on 4:3, making 16:9 even worse. What's stupid is that (as far as I could find while using the UDK) theres no easy option to go Hor+, forcing you to hack it in via UnrealScript. Theres going to be many that wont agree with Epic though, so I think they should just put an option like BioShock. Play they way the dev's want (Vert-) or play the way you want. (Hor+)
Also, the UDK is possibly the only reason I don't actually completely hate Epic right now.