Apple's share of tablet shipments fell from 66% in Q2 2012 to 50% in Q3 2012

"Worldwide tablet shipments totaled 27.8 million units in the third quarter of 2012 (3Q12), according to preliminary data from the International Data Corporation (IDC) Worldwide Quarterly Tablet Tracker. The tablet market grew 49.5% year over year in 3Q12 and 6.7% over the second quarter of 2012. Android shipments, led by Samsung and Amazon, surged during the quarter, at the expense of Apple, which saw its share slip notably during the quarter.
“Samsung took advantage of an opportunity in the second quarter”
"After a very strong second quarter, Apple saw growth slow as both consumer and commercial (including education) shipments declined, and rumors of a forthcoming iPad mini began to heat up," said Tom Mainelli, research director, Tablets at IDC. "We believe a sizeable percentage of consumers interested in buying an Apple tablet sat out the third quarter in anticipation of an announcement about the new iPad mini. Now that the new mini, and a fourth-generation full-sized iPad, are both shipping we expect Apple to have a very good quarter. However, we believe the mini's relatively high $329 starting price leaves plenty of room for Android vendors to build upon the success they achieved in the third quarter."
Apple's slowdown put a sizeable dent in the company's commanding worldwide market tablet share, which slipped from 65.5% in 2Q12 to 50.4% in 3Q12. The remaining top five tablet vendors all gained share during the quarter as a result. Most notable was the impressive quarter turned out by Samsung—driven by its Galaxy Tab and Note 10.1. Samsung shipped 5.1 million tablets worldwide in 3Q12, up 115.0% from 2Q12; that's an increase of 325.0% from 3Q11, when it shipped 1.2 million tablets."

Source:  Press release from IDC, 5th November 2012