There was an interesting writeup on InnoDB adding an integrated memcached component which brings along some of the benefits of a document/NoSQL DB. http://blogs.innodb.com/wp/2011/04/nosql-to-innodb-with-memcached/
This actually seems inevitable and like a step upon the “one DB, pick your SQL/NoSQL depending on the problem” path.
I hope someone (wish I had time to make it me) does a real hard core analysis of the performance differences.
There are applications for SQL based databases and there are cases better suited to document DBs.
Can a SQL DB be retrofitted to get 80% of the document DBs advantages? If so it feels inevitable that that will dominate.
There is simply too many things straight SQL is better for and too much expertise in the world for documents to be the predominate solution.
Maybe its not that easy though.