Interesting! I just had this conversation with a fellow lisp hacker yesterday. What are the alternatives to arc? Or more precisely why not choose one of these other continuations based web frameworks over arc's web framework?
On top of this consider that if you use something like sbcl you'll then be able to use slime and all that it adds when debugging. Also you'll get a number of relatively stable libraries without having to port them to arc.
I think the number one thing plaguing SBCL is multiple threads causing problems. A friend mentioned that hashes should be threadsafe now. That is a move in the right direction. I know I've had problems with threading in SBCL in the past, but that was years ago.
I have thought about what my back up is if I run into too many problems in arc or lisp. Especially if I needed to hire people or contract work. I have my own ideas for how to address such problems, but I think it should be kept in mind when making a decision as to what framework and language to use.
There are public lispy web apps out there. I just don't have a list of them right now. Notably reddit was written in common lisp. I believe sbcl was used, but not sure. Later on it was rewritten in python. The reason is undisclosed but if I had to guess I would say issues with threads.