There have been many famous and subsequently embarrassing impossibility statements in the past, including Lord Kelvin’s 1895 statement that “heavier-than-air flying machines are impossible” (he also predicted in 1897 that “radio has no future”), and the New York Times’ famous 1920 criticism of Robert Goddard and his rocketry experiments, which claimed as obvious that rockets could never operate outside the atmosphere as Goddard proposed, since the vacuum of space would give them nothing to push against! The Times published a “retraction” in 1969, the day after Apollo 11 landed on the Moon.
People who are aware of statements like this (and there are plenty more, this page collects many of them) tend to be wary of making technological impossibility statements, or at least to put them in more qualified terms, based on what is possible today, or at a reasonable cost, or within the known laws of physics, “possible but highly unlikely,” etc. Some examples of such “qualified impossibility” statements (related to human colonization of Mars) can be found in a recent opinion piece on SpaceDaily.com. Anthony Kendall made some really good points about this in his blog. Space advocates may sometimes need to be reminded of important realities, but we should try to not assassinate the messengers or their dreams in the process. I think space is too important to exclude anyone who is supportive of the goals, even if they are not rocket scientists.