At this point I don't think there much you can do.
All iPhone apps MUST be purchased via iTunes, and tied to the iTunes account. There's nothing you can do to change this other than jailbreaking the phone, which is obviously not reasonable for most people.
Unfortunately, Apple has decided to take an extremely restrictive approach to third-party development. The iPhone is very different from other platforms in this regard, and users coming from other platforms who are used to how things work under Windows Mobile, Palm OS, Symbian, etc, are going to find that there's a lot of stuff that simply CANNOT be done, no matter what.
"I am getting a lot of pressure to start supporting the iPhone in my corporate environment."
It's important for end users to understand that Apple retains absolute control over app distribution and availability. Concerns or issues regarding this should be addressed to Apple, since you have no control.
My suggestion: require iPhone users to manage their own software.