If properly set up, the enterprise can set up an encryption key that protects their mail from the rest of your system. They'll have to invest in the effort of setting that up though. With that, the Email part would be almost as secure as a blackberry. Hmm. Just thought of something. Are they using BES or BIS (Internal vs carrier) management? If BIS, there's likely little difference in security. If BES, you're going to have a hard time with the convincing. For a corporation using BES and properly managing their devices, it's simply a system that can't be beat with what Apple offers today.
You might be able to convince them that iPhone is secure enough, but I don't think you can convince them that it is just as secure. Not if they know their stuff.
There's indications of Apple stepping up their security program, but RIM got it right years ago.
"Do the Right Thing. It will gratify some people and astound the rest." - Mark Twain