In a video interview, Char Sample, CERT security solutions engineer, highlights security problems in clouds and suggests that the search for efficiency and cost reduction may lead to the cloud being prone to attacks, so that cloud migration may not be the right solution for the long term (read this paper for a cost-benefit analysis of cloud migration in the long term).
Sure, security is a big issue, but consider also the following two arguments.
First, are you able to provide more security in your own environment than what the cloud provider does ?
Second, there may be other remedies to poor security, e.g., negotiating SLAs with adequate performance bounds, or underwriting an insurance policy (read this paper suggesting the adoption of insurance against network failures).
Watch the video (9 minutes long)