Much has been said about Google App Engine and portability, openness etc. I think that we'll eventually get the same kind of service on an open stack, albeit at a higher cost (higher than free). Aside from authentication, and you don't have to use Google's for App Engine, it's the database that locks you in. Same with Amazon. Someone will build a similar service on CouchDB (should be 1.0 soon enough) with options for your favorite language/framework above that with ease of deployment more like Google's than Amazon's. CouchDB is particularly intriguing because it should be easy to move/replicate the data from one provider to another. Wanna move? Run in parallel, replicate the data, shut the old site off, replicate again. Done. It's even cooler when your site design replicates too. Just put those design documents in the document database. People have been doing this with Lotus Notes for decades.
Don't lock us in. Leave the door open and compete on service, value and reputation.