UPDATE: Apple on Monday issued a press release announcing that it sold its 1 millionth iPhone 3G on Sunday. News from the company has apparently overtaken Gene Munster’s analysis, below.

“iPhone 3G had a stunning opening weekend,” said Steve Jobs, according to the press release. “It took 74 days to sell the first one million original iPhones, so the new iPhone 3G is clearly off to a great start around the world.”

Apple offered no further details. In a revised note issued later Monday morning, Munster attributed the discrepancy, at least in part, to how Apple records its sales. See here.

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In a report that will disappoint investors who were hoping for a 1-million-unit weekend, Piper Jaffray’s Gene Munster estimated on Monday that Apple and its partners sold less than half that many iPhone 3Gs worldwide in its first three days on the market

[Update: In his revised report, Munster acknowledged that “First weekend estimates exceeded our 425k estimate based on strength in international and improvement in activation process throughout the weekend.”]

Still, sales were 40% higher than last year, according to Munster, who [originally estimated that it would] take Apple 17 days to sell 1 million iPhone 3Gs. Last year the company announced the sale of the 1 millionth iPhone 74 days after its first model went on sale.

“The improvement is driven by two key factors,” Munster writes. “1) international availablilty in 21 countries, and 2) a 60% lower entry-level price point.”

Piper Jaffray’s report to clients — the first detailed analysis of iPhone 3G sales — was based on observations of Apple (AAPL) and AT&T (T) stores in New York and Minneapolis, estimates of the rates of sales, the hours of operation and the number of units supplied to each store, as well as surveys of 283 attendees. It’s packed with useful information. The highlights:

• Roughly 425,000 iPhones sold worldwide in three days, despite activation problems [Update: Make that 1 million, per Apple’s press release and Munster’s revised report.]
• 225,000 sold in the United States, 75,000 in the United Kingdom, an average of 7,000 in each of the other 19 countries [Update: Munster now estimates that 400,000 iPhones were sold in the United States, 250,000 in the United Kingdom and an average of 18,000 each in the other 19 countries.]
• 380,000 sold in the first two days (compared with 270,000 in a day and a half last year) [No update yet.]
• Sales slowed by the approximately 15 minutes it took to activate each phone (sales at Apple stores last year took about 1 minute each)
• 66% of customers purchased the 16 GB iPhone (last year, 91% bought the top-of-the-line 8GB model)
• 38% were upgrading from the original iPhone
• 39% were PC users (versus 25% last year)
• 38% of customers in the United States were new to AT&T as opposed to 52% last year

Below the fold: details from the Piper Jaffray survey and its country-by-country breakdown of estimated sales.