Wall Street’s best and brightest are expressing varying degrees of shock at the news Thursday that Research in Motion (RIMM), which delivered 500,000 BlackBerry PlayBook tablets in the quarter that ended in May, shipped a dismal 200,000 in the August quarter — suggesting that most of those first half-million units are still sitting, unsold, on store shelves.

“We believe the PlayBook is poised to follow HP’s TouchPad as the next casualty of iPad’s tablet dominance.” wrote Ticonderoga’s Brian White in a note to clients Friday, a reference to Hewlett-Packard’s (HPQ) decision to pull the plug on its tablet after the TouchPad failed to make a dent in Apple’s (AAPL) market share.

In a similar vein, J.P. Morgan’s Mark Moskowitz noted that Motorola Mobility’s (MMI) tablets had also failed to gain much traction before Google (GOOG) offered to buy the company. And having had a chance to play with Sony’s (SNE) forthcoming tablet entry, he doesn’t hold much hope for that one either.

“We had the opportunity to demo a tablet from Sony … before today’s official launch,” he told clients in a Friday note. “We were not impressed. To us, Sony’s tablet lacks the refined, sleek feel of the iPad and its bezel-like back is not user-friendly.”

Moskowitz dismissed recent speculation that Apple could release an iPad 3 before the holiday season.

“Our latest research continues to indicate that there is no such device slated for production this year. In our view, Apple should be in no rush. The other tablet entrants have stumbled so far, and that trend-line could persist deep into 2012.”