Continuations are more general than lightweight processes. They can also be used to implement exceptions, coroutines, and Ruby-style generators, and many other interesting control abstractions besides.
I'm just curious why we'd explicitly be re-implementing lwp over and over for each unique use rather than having a more formal implementation.
It's great that the arc application server does this for us, but maybe it could be part of the language? It certainly gives a lot of flexibility and it's a no brainer for making programs shorter, since any sort of server would be rewriting it (and probably with many uniquely wrong choices).