In my experiences it has generally come down to two things. Education and control. Our company is involved mostly with business to business customers with a few colleges and hospitals thrown in the mix. If the network folks are not initially involved in the idea and VC is new to them its like anything, thier space is being invaded and you are indeed the enemy. I learned the hard way OFTEN when video over IP came onto the scene. Not only was I asking them to put something that in thier mind would suck the life out of thier network, but I am also a female asking them to do this

. (talk about having credibility issues...) Personally I don't like having to fight my way on the network so I began making sure that the networks folks were aware of "the plan" prior to any installations and ensuring that they were educated on H323.
For those that insist on being difficult do the only thing you can....shake ya tailfeather!! :P