We actually have many customers who do this on the Cloud Servers platform and it works very well. It gives you a flexible location to run your database servers that can scale up/down as necessary and setting up replication in a cinch or even fail-over.
One thing to keep in mind as someone else already mentioned is latency that will be introduced if your web/app servers are not on the Rackspace network. This is still fine for many use cases, but others where you need high performance between your web/app servers and the database it may not be a replacement for your existing DB server, but maybe a good DR solution. However, if you have dedicated servers at Rackspace or an app on Cloud Sites and you need more control over MySQL, this is an excellent option that hundreds if not thousands of customers already take advantage of.