Piper Jaffray’s Gene Munster issued a report early Monday that may draw some attention away from Steve Jobs’ health and put it back on the bright and shiny object Apple (AAPL) released last Friday.

Munster and his team spent the day counting heads and conducting interviews with customers buying the new iPhone 3GS in New York City and Minneapolis. Among their findings:

• 750,000 iPhones. Munster estimates that Apple sold about 750,000 new iPhones over the three-day weekend, 50% more than his initial prediction (500,000) but 25% less than the 1 million iPhone 3Gs Apple sold on launch last July. It took Apple 74 days to sell 1 million first-generation iPhone and three days to sell 1 million units of the iPhone 3G.
• Shrinking windfall. Among the 256 customers surveyed, 28% were switching carriers to AT&T (T), down from 38% last year and 52% in 2007. AT&T’s iPhone windfall is shrinking.
• Brand loyalty. 56% were upgrading from an old iPhone, up from 38% last year. “We believe this shows Apple is developing brand loyalty not enjoyed by other mobile phone makers,” Munster writes.
• 16GB sweet spot. 43% bought the high-end 32GB iPhone 3GS, down from the 66% who bought the high-capacity model (16GB) last year and the 95% who chose 8GB over the 4GB when the first iPhone went on sale.
• Business users. Among customers buying their first iPhone, 12% were switching from a Research in Motion (RIMM) BlackBerry, up from 6% last year. This, says Munster, “may indicate the company is making headway among business users slowly adopting the iPhone platform for corporate use.”

Munster maintains a buy rating for Apple with a price target of $180 a share. The stock closed Friday at 139.48, up 2.6%, before the Street learned that Steve Jobs is recovering from a liver transplant.

UPDATE: Four hours and fifteen minutes after Munster issued his report to clients, Apple announced that it had actually sold more than 1 million units of the iPhone 3GS by Sunday, selling as many iPhones in eight countries as it sold in 21 last year. See here.

See also:

• Munster: 500,000 iPhones this weekend
• Live from the (relatively sedate) iPhone 3G S launch
• Inside Steve Jobs’ liver transplant