Haven't heard of anyone outsourcing the DB layer the first question would be why, is it for convenience, performance etc.?
Also you would want to make sure that the latency between where your DB is hosted and your webservers is relatively low otherwise every request you make is going to traverse the latency and cause slower page loads for your end users.
I've heard of, but not used ClearDB. The benefits are management, redundancy, and replication. If you can manage your own cluster, you won't benefit as much as someone who would prefer or needs someone else to do it for them. The cost is pretty hefty if you have a sizeable database, but then again, if your database is 5-50GB in size, you may prefer to have someone manage it for you as you'd be spending a little time tweaking and tuning MySQL for that type of growth.