I just finished reading the Sitepoint article at:
http://www.sitepoint.com/blogs/2009/06/03/appjet-discontinued/. The AppJet example seems to highlight the inherent risk of building your business on proprietary cloud platforms. Although AppJet offers some suggestions for porting your app to another server (http://appjet.com/hosting), it seems that building anything beyond experimental apps and services upon these platforms is extremely risky. Is the trade-off of easier maintenance and faster time-to-market worth it?
Perhaps the approach of selectively outsourcing components of your application to a cloud service provider with a real SLA is the better model (as people currently do with Amazon).