Look at Google Reader or more recent iGoogle. Service provider decided to serve you or not. If you based on one particular technology or feature – then you are in risk. That service or feature might be shut down at any time because provider is not interested in such service anymore. As well as the API. Interface to cloud services might vary and you need to control these changes to keep your business running.
So. Please combine all risks and benefits and decide if you really need public cloud. If so – please backup.