I listened to Paul Miller's podcast with Rackspace's president of their Cloud Division Lew Moorman this morning. I mostly agree with his comments on easy portability between Rackspace cloud services and Amazon's EC2.
I have not yet used Rackspace's cloud offerings, so my comments here are based on their documentation and a conversation I had with one of their support engineers (for one of my steady customers: I declined some work tasks to move to Rackspace because I don't like to spread myself too thin: I spend a lot of effort staying up to speed on Amazon and AppEngine, so I prefer to specialize on those two deployment platforms). The advantage of Rackspace is the binding of a persistent disk volume with their virtualized server instances (really, they offer a standard sort of VPS hosting service) where with Amazon it takes a little extra work to manage EBS volumes separately. For me, I like the benefit of Amazon's SQS, S3, and Elastic MapReduce - that said, I would make a small bet that Rackspace will provide similar services, otherwise they will just be competing in the VPS business space, although with good support (that said, I have always received very good support from smaller hosting companies like RimuHosting).
Lew Moorman was critical of Google's AppEngine because it depends on very proprietary software (mostly their highly scalable non-relational datastore). A fair criticism, but if you really wanted to move off of AppEngine, there are migration paths like using DataNucleus JDO support for a relational database server or something massively scalable like HBase (from the Hadoop project). One big win for using AppEngine is that if you decide to build on top of the Wave platform, software agents at least for now need to be hosted on AppEngine, and this process is fairly simple.