What makes an OLTP database an OLTP database? This is a pretty broad question.  I know the definition of OLTP.  But it seems like unless you have an environment that has a clear distinction between OLTP and OLAP, in the sense of a database that handles DML transactions and a replicated database that handles the reporting, then this is a bit of an abstraction.

Would it be safe to say that most production databases are OLAP and OLTP?
I would say that most production databases are OLTP with a little OLAP flavor. In a lot of shops the OLAP side of the world is getting more specialized and they really are performing the analytical functions outside of the normal application database. Are there shops that still plunk everything into one database? Of course. But as the capabilities improve (and Denali is no exception), I think you'll start to see a much more obvious divide.