The Williams Institute sent out a press release summarizing the findings:
The main aspect of the report is that it uses an incredibly large sample size of 121,290 adults 18 and older, taken in phone surveys between June 1 to September 30, 2012.
Note that there are really important and surprising results here, namely that the LGBT population is younger, lower-educated and less white than the general population. It is very interesting to me that the racial or ethnic group that is the most likely to respond affirmatively to a pollster that they are LGBT are African-Americans! It's also notable that the youngest co-hort (those under age 29) are the age group most likely to respond that they are LGBT (6.4%), far greater than the oldest cohort of 65 and older, where only 1.9% will say they are LGBT and also has the largest cohort of people who refuse to answer the question (6.5%).
The most important point to take away from this study is to realize that of course the number of 3.4% is at most a lower-bound for the percentage of American adults who are actually LGBT. All this number tells is what percentage of people would be willing to tell someone over the phone who knows their name and number that they are indeed LGBT.
That being said, while some people will quibble with the overall number of 3.4%, clinging to the widely disputed statistic that 10% of adults are LGBT attributed to the Kinsey Report, in my opinion it is very important to celebrate the usefulness of study and analysis of the demographics of the LGBT population.