The iPhone 4’s widely publicized antenna problems cost Apple (AAPL) some sales this summer, but not nearly as many as its exclusive deal with AT&T (T).

That’s the major finding of a survey released Wednesday by Piper Jaffray’s Gene Munster. His team interviewed 258 cell phone users in downtown Minneapolis to gauge the impact of the iPhone 4’s signal attenuation issues. Although the surveyors didn’t ask about Verizon, the subject came up on its own accord.

Munster’s conclusion: “The antenna issue is removing upside potential for iPhone units, but Verizon is actually the most significant factor limiting demand.”

The details:

• Antenna problem: 177 respondents, or 69%, indicated that they were aware of iPhone 4 quality issues. Of those who are aware of the antenna issues, 20% indicated that they impacted their purchase decision.
• Verizon (VZ) problem: For every one respondent that acknowledged the antenna issue about three complained about the iPhone not being on Verizon. In other words, the lack of an iPhone on Verizon is holding sales back by about three times more than the antenna issue.
• Worst case scenario: “In order to quantify a worst-case scenario, we estimate that 40% of all iPhone sales are domestic (was 38% in the June quarter). We are modeling for Apple to sell 11m iPhones in the Sept. quarter, implying about 4.4m domestic sales. If Apple lost 20% of those sales, or 880k units, it would reduce our EPS estimate in the quarter by $0.11, from $3.72 to $3.61.

Munster notes that despite these concerns, Apple continues to sell iPhone 4s faster than it can make them.

Below the fold: The survey numbers.

[Follow Philip Elmer-DeWitt on Twitter @philiped]