Information about the previously unexplained Screamer Boss Infected has been found in the L4D game commentary.

The commentary explains:

‘The Screamer was a boss zombie who didn’t have any actual attacks; he was bound in a straightjacket. You knew when he was around because of his crazed, maniacal cackling. The trick to the Screamer was that if he saw you, you had a moment to kill him before he ran away. Once the Screamer got away from the Survivors to a hiding place, he would emit a loud howling scream that would cause a huge mob of zombies to attack the Survivors.

 

The ‘Screamer’ was cut and replaced with the Boomer

While there were several exciting moments of knowing you had to chase him down to shoot him before he screamed, dodging zombies all the way, ultimately it proved too confusing for Survivors to discover how he worked, or even to reliably notice him in the crowd. He was cut, and his horde-drawing attack eventually evolved into the Boomer attack.’

So it would appear that the Screamer previously discovered in the L4D files as concept art was scrapped and replaced with the Boomer. Maybe one day int he future we shall see his return?