Your run of the mill virus, even if it affects a guest operating system hosted on a "cloud", will not be able to impact anything other than that guest.
Given how much different systems "talk to" each other in order to coordinate important functionality like failover and live-migration, though, it definitely seems possible that a highly-sophisticated attack could be devised to bring down an entire "cloud" in one fell swoop.
Fortunately, there are some pretty big brains working on defending against these types of attacks. Unfortunately, there are equally intelligent people on the other side of the issue.
In reality, the level of risk should end up being about the same as the level of risk inherent to any peice of software. The "cloud" community will see security updates, patches, and the occasional exploit.