I think it will come down to four things: Who buys the phone; whether IT thinks its secure enough; whether it will integrate well enough; and whether IT trusts and has relationships with the vendor (s).
The Phone Buyer
This is the one area that I think favors Apple initially. Right now employees actually make the decision to buy the phone and their company simply reimburses them for some or all of the charges.
This is different in RIM accounts, but as well penetrated as RIM is in the enterprise they are far from the dominant phone supplier. So they dont really get that much of a vote.
Apples primary and overriding focus is on the buyer and cool technology that pulls those buyers in like bees to honey. Folks lust after iPhones and, at least for the moment, no other phone creates the unbridled lust that this device creates.
Recently I was on a panel at the Information Week 500 conference and the CIO that shared the panel with me indicated he would like to give all his employees iPhones and this was a Fortune 500 insurance company and Microsoft shop.
Unless the Google G1 phone can close this lust gap or change this one dynamic, the iPhone has a significant advantage here.
The iPhone has had a number of security problems and the G1 is mostly untested with regard to security.
Both these phones are likely to be viewed as relatively unsecure, though the G1 largely uses online resources. This suggests the security problem it probably has is largely mitigated because the phone doesnt actually contain much confidential information (if deployed properly).
The G1, deployed properly, is basically a terminal to enterprise applications through the browser and as long as proper access security is used it should be competitively secure. On the other hand the iPhone is not only a more traditional phone, it is connected to MobileMe which pulls enterprise mail into an Apple service which hasnt been working that well.
Even the idea of another company getting mail from an internal source, particularly an executive, is likely to make some security folks catatonic. MobileMe potentially requires, in a secure environment, a security audit, This is because its a repository for confidential information and if it isnt secure could provide sensitive information about anyone, both personal- and job-related, to the world.
This means that while there doesnt appear to be a differentiating advantage between the two devices both are relatively unsecured the MobileMe service would likely fail the iPhone in secure environments.
Apple simply doesnt play well with others and that is likely one of the reasons MobileMe isnt working out very well.
However to integrate into IT systems you have to be able to work closely and honorably with people you might otherwise view as competitors. Google is vastly better at this, given that their tools cross a wide variety of vendors who preload them on PCs today.
Because they have to generally run on Microsoft platforms they seem to know how to develop products that function there and have more than a passing familiarity with UNIX and Linux. While not as good at integrating as Microsoft currently is, they are vastly better than Apple is. That gives them the lead in this category as well.