Bert;The issue is as Alex states above, that there shall be no law regarding the institution of a state religion etc. etc. etc...
It's a dodgy issue at best, as it all comes down to a matter of opinion for the most part... at what point does requesting a 'moment of prayer' go from being a personal gesture and become a governmental declaration? I suppose that Bush's prayer would all be considered 'personal gesture', as long as he does not try to mandate mandatory prayer via executive order or what have you.
M