I tried out the Google App Engine SDK the other day, and unfortunately I wasn’t really thrilled with it. I’m not well versed in python, so the barrier to entry for me was higher than I’d hoped.
When I learned that the data storage mechanism isn’t standard SQL, I gave up! Hosting with LAMP (linux, apache, mysql, and php) is easy and ubiquitous, so hosting with Google isn’t really much of a difference.
With MySQL, its pretty easy to replicate databases so you can achieve high availability, and you can then use something like Amazon’s S3 or even Picasa to provide a reliable file storage mechanism. If that’s not your cup of tea, you can use something like MogileFS.