Funtimes: You can modify default gravity in WillowGame.ini in My Documents/My Games/Borderlands/WillowGame/Config
Just search for DefaultGravityZ=-500 - it should be under Engine.WorldInfo. Lower settings can make you jump extremely high. I haven't tried positive settings, assuming it would make everything useless since you'd just keep floating up probably. For a non-annoying but still interesting and sometimes convenient (read: pretty cheatish/travel time decrease) change, I'd suggest -200 to -150, which is what I'm at now.
I did some testing - it works over lan so that if you're hosting the game, the other players get the changed settings, but if they're hosting, it returns to normal. So it's essentially a server-side tweak. I don't know what it does over GameSpy yet - I'm using a copy of my partner's game to play with him, so I'm wary of playing online lest he get banned.
Also, under [WillowGame.WillowInventoryManager] in the same file it appears you can alter the inventory and weapon slots you get at the beginning of the game. I tried putting it to 5 after getting 4 slots - it's buggy, but you can -kind- of make it work. I can switch between five equipped weapons after dropping and picking up one, using the scroll wheel. There's just no UI support for it, so the only way to unequip a weapon in the fifth slot is to drop and replace it from the ground with another.
Still, kinda useful if you don't want to go through your first playthrough with a tiny amount of weapon slots to play with. I know I was constantly using the inventory to change out to other guns, which kinda got aggravating. I kinda wish they had given you more to use.