Piper Jaffray’s Gene Munster and his colleagues called 25 Apple Stores in the U.K., Germany, France, Japan, and Australia over the long weekend, 14 on the day of the launch and 11 the following Monday.

Results:

• On Friday, 29% of the 14 stores called had sold out of either all Wi-Fi or 3G models
• On Monday, 36% of the 11 stores called were sold out of either Wi-Fi or 3G models
• By comparison, Piper Jaffray’s checks of a sample of U.S. stores 48 days after launch found 74% were completely sold out of all iPad models

Munster’s conclusion:

“We believe the iPad is a long-term investable theme and a reason to buy shares of AAPL, but we caution investors to keep iPad expectations in check given our domestic and international checks suggest supply is low.”

Still, based on Apple’s announcement Monday that it had sold 2 million iPads in less than two months, Munster is finally raising his unit sales estimate for calendar year 2010, from 4.3 million to 6.2 million. That 44% tweak added 25 cents to his earnings per share estimate for 2010.

We expect Munster won’t be the last Apple (AAPL) analyst to issue new iPad sales estimates in the days ahead. See here.

[UPDATE: Three hours later, Kaufman Bros.’ Shaw Wu increased his iPad estimate for the June quarter from 2 million to 2.7 million and for calendar year 2010 from 6.3 million to 8 million. For more hasty revisions, see here.]

Below: The details from Piper Jaffray’s weekend phone calls, which make it clearer that most stores still have iPads in stock.

[Follow Philip Elmer-DeWitt on Twitter @philiped]