According to the official release on the subject;
Mad Moxxi's Underdome Riot is an intense single-player or cooperative experience that expands the mayhem by adding three new Riot Mode arenas where players will endure an onslaught of Pandora's baddest enemies. Working together, players will fend off the masses as they fight for fame, honor, and more importantly, loot. Mad Moxxi's Underdome Riot also adds a new bank feature, which allows players extra storage capacity for when they encounter one of the more than 16 million weapons that Borderlands has to offer; and the ability to acquire two additional skill points as quest rewards, making their characters even more powerful than before.
Riot mode, according to Gearbox CEO Randy Pitchford, is "like Smash TV in coop FPS, but in the Borderlands". Sounds good to us, and if you're too young for Smash TV, think Halo 3's Firefight mode... i think i just did a small sex wee.
ps: http://futureradio.co.uk/ now has a new radio player. listen, and browse you dogs! they just dropped our linkage on teh air, we are mediastars! -haha-