"On our side: if Sparrow was to do Push today, we would have to store your credentials (login/password) on our servers to frequently poll your accounts, and send you notifications.
This is a responsibility we're not ready to take. As a startup focused on iOS/OS X development, we do not have the skills to secure your data on our servers and we do not want to put sensitive information at risk."
I've just downloaded the app and it's awesome from a UI perspective, but it's never going to be an option as my primary mail app in it's current form which means I really can't recommend it to anyone.
Yes Apple might change their policy on this but it seems unlikely, at least in the short term. I think their best hope would be a new mechanism that allows something like this in OS6 rather than Apple just going "OK" but pragmatically they need to be looking at what they can do rather than crossing their fingers and waiting for Apple.
But that's not really my point. My point is that petitioning Apple likely isn't a productive route for them to follow, they need to look at how they address this problem without relying on a third party who are unlikely to accommodate them.
Gmail has mechanisms for allowing access without you having to share you credentials with another organisation so it can be done. I get that it's not their core competence but if this is a market they want to be in, they might need to start expanding their skills base.