The spell target must be friendly, in line of sight, and in range of the caster.
If the target is flagged PvP, the caster becomes flagged.
The target gains the buff.
Group buff splash - With some exceptions†, all characters in the target's party, including pets, that are within 100 yards of him also gain the buff. This effect is not limited by line of sight. As a result, the caster can buff people at triple the spell range.
† Characters will be excluded from the group buff splash if they are lower than the spell's trained level, or are flagged PvP when the caster is not.
Exclusion by PvP status disrupts raid preparation; typically, the raid will wait for flags to expire, or order the buffing classes to become flagged themselves.
This section concerns content exclusive to Cataclysm.
With Cataclysm, [Divine Spirit] and Prayer of Spirit will be removed from the game. As Spirit will be the primary mana-regeneration stat, Blizzard doesn't want it to vary as much between solo, small group, and raid play. [Blessing of Kings] and [Mark of the Wild] will not boost Spirit either.