Barry Arrington at Uncommon Descent offers a simple three step rejoinder to it:
Step 1: Assume that Craig Venter (a biologist working on developing artificial life) succeeds in developing an artificial life form and releases it into the wild. Step 2: Assume that a researcher - let’s call him John - later finds one of Venter’s life forms, examines it, and concludes that it was designed by an intelligent designer.Arrington goes on to ask, "Does it say anything about the paucity and/or weakness of our opponents’ arguments that they think the 'Who designed the designer' argument is one of their best?"
Step 3: John’s design inference is obviously correct. Note that John’s design inference is not any less correct if he (a) does not know who Craig Venter is; and (b) is unable to say who designed Craig Venter.
It's the fundamental claim of intelligent design proponents that certain aspects of the world, specifically cosmic fine-tuning and complex specified information in living things, are evidence of a cosmic intelligence. Despite what some of their philosophically unsophisticated critics may think, their argument is not the least discredited because they can't say how the engineer came to be or who the engineer was (or is).