Ben is correct. Apple's policy on this is pretty clear. Anything that comes from the App Store requires an Apple ID authentication to get installed, free, paid for, whatever. Even with the "push" process allowed now with iOS5 it still will prompt the user to enter their ID (or some ID)
I think the thinking here is, Apple does not want apps pushed to users surreptitiously, probably for security reasons but also because Apple still strongly views iOS devices as 'self managed', not controlled by the enterprise. Despite this, iOS devices are making inroads into the enterprise in droves. The hope is Apple will eventually allow more control over how this all functions, but for now, their focus is firmly in the perspective of the consumer.
As Ben also noted, in-house developed apps that don't come from the App Store are exempt from this restriction. But that is the only case I know of.
