"Arc's server doesn't actually use continuations - it uses functions that serve as continuatinos. There's no call to 'ccc or any of the wrapping forms in srv.arc (at least none that I remember)"
That's right, no 'ccc. That's why I could implement it so easily in Lua (no real continuation in Lua either). As for TCO, I'm not sure it's a real problem in this case. Sure, a lot of simple functions are written in a recursive style, but they can be trivially rewritten in an iterative style.