According to Justice Kennedy, prayer that is "solemn and respectful in tone, that invites lawmakers to reflect upon shared ideals and common ends" is okay. (I suppose in future court cases we might look for the high court to become arbiters of what constitutes "solemn and respectful tone.") On the other hand, prayers that do things like "preach conversion" are not okay.
That's one thing about government sanctioned prayer. In the end the government starts telling you what to pray and how to pray it.