What usually gets me in trouble is my response -- enterprises are already using cloud storage. If you speak with many of the cloud storage service providers, a large chunk of their services are being charged on Fortune 100 company credit cards.
Certainly this does not count as IT endorsement, but it does remind me of the early days of the PC when people started lugging their Compaq "portable" to the office because they liked using Lotus 123. In those days, the statements made by the guys in the white room were similar to what you hear today about cloud. IT staffs that simply dismiss it are like those IT staffs of the early 80s dismissing PCs.
I know, you're never going to trust your data to be stored on the Internet. Yeah, and who trusted the 5 MB hard disk from Radio Shack either? Just as hard disk technology matured, so will cloud storage and cloud computing. It will become reliable and cost effective.
The challenges facing broad enterprise adoption now, however, are real. For example, using the cloud for data that needs to be stored because of compliance or regulatory issues is a big concern, and those concerns may be better served by an internal archive storage system or building an internal cloud.