[Twisted-Python] I could swear I've seen this pattern *somewhere* in Twisted...

[James Y Knight]

How about this? sem.run simplifies things a lot.
Note also consumeErrors=True to DL.  You probably always want that,  
as it causes the errors to not get propagated to the toplevel error  
printer, but rather only reported by the DL.

def parallel( iterable, count, callable, *args, **named ):
     sem = defer.DeferredSemaphore(count)
     dl = [sem.run(callable, item, *args, **named) for item in iterable]
     return defer.DeferredList(dl, consumeErrors=True)

James