Both Amazon and Google (and probably others as well) provide web pages which monitors its service status. The one which I go to, when I need to compare availability and to detect service problems is the one called Cloudstatus by Hyperic.
They try to monitor most of the individual services provided by Google (Engine, Datastore, Memcache, [...]

If you are like me, and not attending Velocity 2009, you should track this page for the presentation slides from this years conference.
 
 

2 Years Later, Loving and Hating the Cloud
Death of a Web Server: Crisis in Caching
Fixing Twitter: Improving the Performance and Scalability of the World’s Most Popular Micro-blogging Site
Hadoop Operations: Managing Big Data Clusters
Introduction [...]

Last week I spent a few hours building a search engine testing tool called “BlackboxSET”. The purpose of the tool was to allow users to see search results from three different search providers and vote for the best set of results without knowing the source of the results. The hope was that the search engine [...]

Most web applications needs at least the following services to be self sufficient. Computational power, storage, webserver/cdn, database,  messaging, loadbalancer and monitoring.
Here is the tried and tested steps as recommended by AWS folks

Move static web content to S3 storage first. Images, css stylesheets, javascript files, html, etc can all be moved to S3. Its easier [...]

I heard a great set of Google App engine datastore related talks at the google I/O conference. I think this is one of the best out talks I heard which is now on Youtube. You should watch it if you are working with or planning to work with Google App Engine in the near future. [...]