The Spontaneous Summoner feat from Complete Divine lets you spontaneously cast summon nature ally as if you were a druid. One of the conditions to get this feat is being able to cast SNA as a normal spell.
It is not extremely clear to me whether this feat applies to all your classes or only to the classes from which you can cast SNA as normal spell, more specifically: can a druid/sorcerer with this feat use his sorcerer slots to spontaneously cast SNA because he meets the feat prerequisites from being a druid even though he can't cast SNA as a sorcerer?
I am inclined to say no because the feat specifies
You can spontaneously cast summon nature's ally spells (from your class spell list)
But to me it looks like a very vague statement and I'm not sure how to interpret it.