While there are a lot of AWS offerings/cloud services which have vendor lockin, the core EC2 API is supported by Eucalyptus (http://eucalyptus.com) and OpenStack (http://openstack.com).
If you're using auto-scaling or beanstalk or whatever, that's an example of AWS offering something where there isn't a norm, isn't a "normal way" to do it, and if you need that thing, your other alternative is to build it yourself - which is still an option should you decide to move away from AWS.
Lockin is avoidable switching cost which benefit the party imposing the switching cost. Are there examples of AWS doing that?
That said, I don't think they set it up this way to specifically cause lock-in, it's just technically difficult to allow users to manually manage replication alongside RDSs automated system.
Same with their cloud search - sooo obviously solr/lucene-based but 0 exposure to the lucene syntax.
Not a big deal though, it's not like it totally locks you in to use these.
Except it isn't. http://aws.amazon.com/cloudsearch/
"Amazon CloudSearch was created from the same A9 technology that powers search on Amazon.com."