To develop FvM's post, it is actually very common for generating sets to use a centre-tapped earth. These may even be used to provide power to installations which are usually supplied from the local supply and this means that their voltages can be unpredictable.
With a centre-tapped earth, the neutral - earth voltage is the same as the live - earth voltage. (and both of these are half the live - neutral voltage).
In places where this DOES NOT apply, then the neutral - earth voltage should be very low, as others have posted, but there are several reasons why it can vary substantially, particularly when the load on the 3 phases is very unequal. The heavily loaded phases will "pull" the neutral voltage towards them. In fact the voltage distribution across 3-phase supplies can become quite complex and we should never make assumptions about neutral being "safe". It isnt.