Is there a quest related to the New Vegas sewers, or are they just something new to explore?

There's at least one quest that sends you into the sewers, that I've run into. Other than that, they're just there to explore, I guess.

There is a arena down there to if you want to check it out.

Haha, bad idea to go to the Thorn, that place is very buggy, with invisible walls, friendlies turn hostile, etc.
I suggest you go to the Fort's Arena instead, although that arena is very lame with only 4 battles availible.

There is also a unique version of the Lead Pipe down there. It's not very good compared to other unique melee weapons, but it's ... well, it's certainly a unique lead pipe.

Thanks, guys.