I'm sure corporate sales haven't changed much since I had exposure - and we were a small business.
The decision makers sit around a table with their carrier rep, and the pitch a deal. Obviously the lowest cost is going to look better. The more of a handset you buy, the better the deal. I'm also assuming the bulk sales margins aren't as in favor of the iPhone as they are of a cheaper droid model.
One interesting thing about the whole process was we had started with GTE mobilenet, which morphed into Verizon Wireless. We were in a professional organization that had negotiated this awesome deal with ATT and so my Dad finally relented to hearing their pitch - not thinking they were going to be able to touch our VZW deal with a 10ft pole.... Not to mention, ATT didn't have any service in over 1/2 our sales area.
He listened, told them thanks but no thanks. They asked why, and he said you are twice what we are paying and don't have coverage. The ATT guy told him no way, and even after being shown a bill showing "yes way" he still insisted it wasn't possible! LOL!
Anyway, if enterprise isn't adopting iPhones then part of the blame is going to go to the sales reps selling it to them.