Thanks to some NPD figures brought to light by Morgan Stanley (via AppleInsider), it's now clear how much of an impact the MacBook Air's mid-2011 update had on Apple's diminutive notebook. Though Apple's thinnest laptop accounted for a mere eight percent of overall notebook shipments in May, just two months later that jumped to 22 percent.
Today, the MacBook Air makes up 28 percent of Apple's overall notebook shipments. Since Apple discontinued the plastic MacBook in July (with the exception of educational sales), it's safe to assume the MacBook Pro accounts for the bulk of the remaining 72 percent of shipments. Because Apple keeps such a close eye on its supply chain, these shipment numbers probably reflect very closely on actual sales numbers.
The deletion of the plastic MacBook from the lineup may in fact account for some of the MacBook Air's newfound success. The US$999 11-inch MacBook Air has supplanted the plastic MacBook as Apple's "entry model" notebook. The vast upgrade it received in July is probably a much bigger factor in the MacBook Air's newfound popularity; by itself the addition of Thunderbolt vastly expanded the device's capabilities, but the addition of i5 and i7 processors arguably made the MacBook Air an attractive standalone Mac for the first time. I even considered replacing my current (out-of-warranty) 17-inch MacBook Pro with a 13-inch MacBook Air, but I decided to upgrade my current machine with an SSD instead.
Apple is likely pleased with how well the MacBook Air has taken off after its last update, and with rumors constantly swirling about a 15-inch addition to the MacBook Air line, it's certainly possible that the Air lineup may eventually take over from the Pro as Apple's most popular notebook model.
Apple MacBook Air 13-inch (early 2015)
Apple MacBook Pro 15-inch with Retina Display (mid 2014)