I took a "Physics of Sound" class when at college, and remember similar statements being made.
This page on the "Sensitivity of Human Ear" indicates that the threshold of hearing is 0 decibels. The threshold of pain is 130 decibels (1013 stronger signal) - or about the volume of a rock concert.
MakeItLouder.com has a chart saying that a mosquito buzzing 10 feet away (where
N is normalized, and the leading number is the decibel measure):
0 (N) BEGINNING OF HEARING, A MOSQUITO 10 FEET AWAY, THE EAR DRUM MOVES LESS -REF.1.1990
THAN 1 / 100 THE LENGTH OF AN AIR MOLECULE (N)
That same chart indicates that a pin drop from a height of 1cm at a distance of 1m is 15dB.
However, that chart also has a gross misstatement about Krakatoa exploding in 1883 that it "CREATED A 3000 FOOT TIDAL WAVE" - at no point was the tsunami measured (above mean sea level) at higher than a couple hundred feet. They also reference an absurdly high ejection maximal distance (34 miles), when it was more along the lines of 17-20 (ref and here) (not that this is an indicator of other misinformation, but it's a warning).
Also, to quote Wikipedia's Decibel article:
The decibel is commonly used in acoustics to quantify sound levels relative to a 0 dB reference which has been defined as a sound pressure level of .0002 microbar, or 20 micropascals. The reference level is set at the typical threshold of perception of an average human and there are common comparisons used to illustrate different levels of sound pressure.
See also example of pressure. The bottom end of the range is in the neighborhood of 2x10-5 Pa, or about 2.9x10-9 psi.
Based on the size of the ear drum alone, less than 1 cm in diameter, it must be extremely sensitive - there's not much place for it to move to start with.