It doesn't forbid the electron from hitting the nucleus. In fact, as Drakkith pointed out somewhere above, there is a small but finite probability that the electron will find itself in the nuclear because its wavefunction (in the H atom at least) overlaps with the nucleus slightly. It does sometimes occur in a process called electron capture. However, the bulk of the probability amplitude lies outside the nucleus. This is no accident, and has to do with the fact that we are looking at a central force problem -- while the central force seeks to confine the electron to an ever decreasing region about the nucleus, the variance in the electron's momentum becomes larger as a result of the HUP.Excuse me if this is off-topic, but I have to ask... How does (or does it) the the uncertainty principle forbids the electron to "hit" the nucleus?