Piper Jaffray recently conducted a small survey of mobile phone users (n=258) in Minneapolis, Minnesota to determine the impact of "antennagate" on iPhone 4 sales. Almost 70% of survey respondents had in fact heard about the problems with the iPhone 4's antenna and 20% of that group said that the issue had "impacted [their] purchase decision."
Given the small sample it's hard to extrapolate from this data reliably. But I know from experience that some people inclined to get the iPhone 4 did "think twice" or have yet to get the device. But far more don't consider it a "dealbreaker."
In addition: 40% of respondents had considered buying an iPhone 4, 29% considered buying an Android phone. Among those who were aware of the iPhone antenna problems cited other issues (cost, carrier) as more significant barriers.
Source: Piper Jaffray (via AppleInsider)
The antennagate issue definitely gave Apple a black eye and may have dampened sales -- though probably not by 20% -- but assuming the company addresses the antenna issues in iPhone 5 it will overcome the episode.