Checks with 50 Apple retail stores found that the iPad 3G remains completely sold out in the U.S. weeks after its release and 7 days ahead of the device's international launch.

Analyst Gene Munster with Piper Jaffray said his team contacted representatives at 50 Apple retail stores and found that 74 percent of them were sold out of all models of the iPad, with the Wi-Fi model available in 26 percent of stores, and the 3G version sold out. It's been the same story for Apple since the iPad 3G first launched on April 30 in the U.S.

To deal with the limited supply, Apple's stores have begun offering customers the opportunity to be placed on a "Notify Me" waiting list, which will have the store hold an iPad when it arrives for 24 hours. One Apple store representative told Munster that customers who enlist in the in-store reservation system typically wait 4 to 7 days to receive an iPad. That's better than the 7 to 10 day wait found via Apple's online store.

Munster said constrained supplies suggest there could be an upside to his projection of 1.3 million units sold in the Apple quarter. But he also cautioned that Apple's inability to meet demand means that some near-term Wall Street projections are "getting ahead" of what the Cupertino, Calif., company can actually supply.

"GIven the international iPad launch on 5/28, and continued strong demand in the U.S., we expect supply constraints to last through the June quarter, reaching equilibrium in mid-to-late Sept. quarter," Munster wrote.

The first day of iPad 3G sales was enough to push the device past the 1 million milestone, with Apple reaching that many sales just 28 days after the hardware was introduced. Apple officials have admitted that they were caught off guard by the strong sales start of the new device.