One of the fundamentals of public cloud is that it uses the Internet for connectivity. Even the VPN solutions use the Internet. Connectivity is limited resource and with the thin margins in public cloud bandwidth is a heavily scrutinized, monitored, and protected resource. Similarly enterprises labor continuously to optimize network architecture and minimize the size of the pipes to the Internet. Enter hacktivism and its favorite tool of disruption, the distributed denial of service (DDOS) attack.
A DDOS attack is basically a flood of requests that hit a targeted range of internet addresses seeking to overwhelm the systems ability to respond. Small attacks take down a server, medium attacks take down a site, large attacks saturate the nework and take down an entire company. Essentially so much garbage is being thrown down the drain that eventually the system blocks up and nothing can get through. When this happens nothing goes in or out.
Imagine a bank, hospital, or any other company who begins to use public cloud for enterprise solutions. To the hacktivists it would be the same as inviting their disruptive methods into the data center. A DDOS attack could essentially take the company off-line unable to complete any transaction involving the public cloud. No more access to systems, data, records, images. I expect this is an issue already faced by salesforce.com and other SaaS providers who become the target because of who their customers are rather than as a result of their own actions. It would certainly make a prospect want to know who else uses the service in advance, but well beyond the concern of shared hardware and co-mingled databases.
I'm sure there are ways to architect around this, however it those will likely increase costs and complexity, the direction opposite the strategy of enterprises. Of course adding this issue to the litany of security concerns in the end only serves to decrease confidence in the public cloud.