  The behavior of an ideal $D$-dimensional boson gas in the presence of a
uniform gravitational field is analyzed. It is explicitly shown that,
contrarily to an old standing folklore, the three-dimensional gas does not
undergo Bose-Einstein condensation at finite temperature. On the other hand,
Bose-Einstein condensation occurs at $T\neq 0$ for $D=1,2,3$ if there is a
point-like impurity at the bottom of the vessel containing the gas.
