Amazon has really impressed me with their efficient and simple cloud solutions. Last week they announced a hosted search solution in their cloud, named CloudSearch. The idea is the same as for all the other cloud services, namely that they take care of all the hosting, provisioning and scaling (CloudSearch can auto-scale) while you concentrate on indexing and searching.
If you want to run Solr but be able to scale it into the cloud, CloudSearch won’t do that for you. Then you should consider using SolrCloud on Amazon EC2 or another cloud platform instead. There are even providers of hosted plans out there such as IndexDepot and WebSolr which take care of the hosting part for you.
I look forward to playing with the service the next time I find myself wondering what to do