Someone over on the official forums for the game wrote a neat little config utility that handles changing some things that aren't available in the game menu (it pretty much just automates making the ini tweaks) - including the FOV settings. There's various options to bind it to the movement keys so it gets re-applied any time you move.
http://gbxforums.gearboxsoftware.com/showthread.php?t=85874
Here's a screenshot:
I've got this hack but its not working. I have the required .NET framework 3.5 and my config files are in the correct place. I'm trying to use this with the Steam version and with all DLCs. Has anybody here had difficulties like this?
Edit: Problem sorted.