Having spent a long time in the business of information security I always get nervous when people talk about shared services; having seen poorly configured 'shared' firewalls allow one customer to find themselves on the intranet of another, sniffers installed on shared networks, and numerous less legitimate/benign activities.
The problems usually stem from programmatic errors, logic breakdowns, or misinterpretation of specifications by developers; this is likely to be the problem with Cloud computing for the next few years; it almost certainly what caused the problems experienced recently with GoogleApps.
Should this put you off Cloud computing? Probably not 10 years ago we had all the same problems with websites. It takes a while to learn how this new stuff works.