Theoretically, the phase shift can be any number of degrees. But in practice, it's 30 degrees in a standard delta-wye transformer. (Well, 150 degrees if you want to be technical.) This is rarely desireable, and just a by-product of 3-phase power.
The phase shift creates problems with UPS systems, as the UPSs generally take delta power and output wye power. The maintanance bypass circuit has to phase with this, and match it exactly. So, where the UPS is operating at a different voltage from the building power and input/output transformers are required, it creates a real engineering challenge to ensure everything matches.
[This message has been edited by SteveFehr (edited 10-21-2006).]
