So I'm having trouble coming to a determination for this for myself and it may become relevant shortly, so I'm looking for outside interpretations. Can a person casting Sunbeam use one of their allotted beams on the same round they cast as part of the casting action, or must they cast the spell then wait until their next turn to use a beam as a standard action?
It'd be the former, given this clause:
For the duration of this spell, you can use a standard action to evoke a dazzling beam of intense light each round.
Casting the spell simply allows you to do the bolded part, for the duration of the spell. It is not like a melee touch attack where you can make a Touch Attack as a Free Action as part of casting the spell.
However, if you had a Greater Quicken Metamagic Rod, you could cast this spell as a Swift Action, and then use the Standard Action within the same round.
That's the conclusion I came to as well, just didn't want to gimp the encounter if I was wrong. Thanks Darksol!