The App Engine team has a proactive policy about posting about downtime:

https://groups.google.com/forum/#!forum/google-appengine-dow...

Since the team highlights basically anything that looks like it is impacting customers, the issues don't always warrant a stand-alone blog post, but you'll notice that generally speaking the last post in each thread is a full public post-mortem with diagnosis and remediation.

Let me know if there's more you think might be useful for you as a GAE customer. Thanks!