Thanks for your answers to my "Questions about Mahout's maturity" post.
Are the algorithms implemented by Mahout all implemented in Hadoop, i.e., does using Mahout imply that the problem can be implemented in Hadoop and is therefore amenable to implementation in MapReduce? As Mahout is about scalability, I suppose this question is getting at whether all of its scalability is, in the end, based on MapReduce.
Also, is there something in the book about running on Google AppEngine (GAE) and Amazon EC2?
Most of it is based on Hadoop / MapReduce, yes. Not all of it is though, in particular a lot of the recommender code, which also has a significant non-distributed presence.
I don't think you can run Hadoop on GAE? Or at least I have not heard that you can, nor tried. I have personally run it on EC2. The book has a few pages on running Hadoop jobs on EC2; it's generally quite straightforward if you understand what's going on when you run it locally.