Piper Jaffray’s Gene Munster, one of the few Apple (AAPL) analysts who believes in counting heads, issued a note to clients Thursday with the results of a quick survey of early Verizon (VZ) iPhone buyers.

He and his team interviewed 40 customers in New York City and Minneapolis. “Lines were shorter than expected,” he wrote. “But recall that online pre-orders sold out in less than a day.”

The pie chart at right shows the predominance of current Verizon customers, followed Sprint (S), T-Mobile (DT) and AT&T (T). Based on what is admittedly a very small survey, Munster concludes that cannibalization of existing AT&T customers may be less than he anticipated.

Among his other findings:

• 8% of buyers were upgrading from an older iPhone
• 28% had been using a Research in Motion (RIMM) BlackBerry
• 13% were switching from a Google (GOOG) Android phone
• 8% from a Nokia (NOK) phone
• 45% from “other,” including so-called feature phones
• 25% bought the 32GB iPhone 4, down from the 54% who bought the high-capacity model at the AT&T launch last June
• One buyer in four (25%) owned an iPad; of the 75% who didn’t, 43% indicated that they would likely buy one in the next 12 months

Also on Fortune.com:

• Why are the Verizon lines so short?
• Verizon iPhone: The reviews are in
• Survey: 44% of Verizon Android users likely to switch to iPhone on Day One

[Follow Philip Elmer-DeWitt on Twitter @philiped]