It was only a matter of time before Amazon's offerings would include an in-memory cache service via their amazon webservices (aws) cloud architecture and today is the day! If you're looking for a scalable object caching system, you might want to seriously consider Amazon's ElastiCache. Although it's currently in beta (what isn't these days), it's open for business and ready to handle your caching needs.
The documentation is farily robust considering it's 'beta' status. I found their section on choosing cache node type and the number of cache nodes particularly helpful.