having managed hosting doesn't mean you don't need to a dba. In my experience all the host does (worked for a managed hosting company) is set up the servers and keep them running, their help really can't help you with tweaking queries or tuning the db. I can't speak for on engine yard specifically. But I wouldn't bank on your host knowing anything beyond the basics of databases.
Engine Yard has several excellent DBAs on staff. They tell you what queries are slow and tune the db to optimize for your type of traffic. That's why they are worth the 30% premium on top of EC2.
engine yard may be an exceptional exception to the rule. I was generalizing, and speaking of managed hosting in general. Like I said, I can't speak of engine yard specifically, I haven't worked for or with them.
