Can you go into more details? This has always worried me about hosting in general - for sites that can't justify multiple servers, or the complexity of setting up MySQL replication, what's the best approach for HA? There's a lot of businesses that fall into this 'important and profitable but not that profitable' range...
Not if you design your application correctly. HA is really not that hard to do, in the cloud or otherwise, so that single points of failure (e.g., vm or dyno outages) do not result in service unavailability. Also, consider that if you're not building HA into your apps, you are still at the mercy of things beyond your control with local physical servers. What if your upstream network provider accidentally publishes a BGP route that sends all your traffic into the void? Or, in a timely example, what if your authoritative nameservers get DoS'd? Build HA apps, aim to physically separate key redundant infrastructure pieces (built in to most big PAAS services and configurable in most big IAAS services), and you'll have no worries using a cloud service.