The Quicken Spell feat carries this special requirement:
This feat can't be applied to any spell cast spontaneously (including sorcerer spells, bard spells, and cleric or druid spells cast spontaneously), since applying a metamagic feat to a spontaneously cast spell automatically increases the casting time to a full-round action.
This is in reference to this rule in the Metamagic SRD:
Therefore, such a character must also take more time to cast a metamagic spell (one enhanced by a metamagic feat) than he does to cast a regular spell. If the spell’s normal casting time is 1 standard action, casting a metamagic version is a full-round action for a sorcerer or bard.
However Rapid Metamagic specifically supersedes this rule:
When you apply a metamagic feat to a spontaneously cast spell, the spell takes only its normal casting time.
Can Quicken Spell be used spontaneously if the casting time is reduced by Rapid Metamagic?
The way I read it, Quicken Spell's special is more of an answer to a logistical question, rather than an explicit prohibition, given that it sites another effect(the spontaneous spell's increased casting time)as the reason for disallowing it. I see it similar to if the PHB said, "You can't wield three weapons since all playable races have only have two hands." If you somehow gained a third hand, the rule would cease to apply.