Károly Négyesi (aka chx) talks about MongoDB and NoSQL databases. He's working on building a MongoDB module that integrates with Drupal 7's FieldAPI pluggable storage to make Examiner.com more scalable. He's also working on a MongoDB DBTNG driver for Drupal 7.
Négyesi mentions Brewer's CAP Theorem, which states that distributed systems can only choose two of the following three options of Consistency, Availability or Partition Tolerance. MySQL has a relational data model and has chosen Availability and Consistency while MongoDB has a document-oriented data model and has chosen Consistency and Partition Tolerance. Below is a graphic from Nathan Hurst's blog that visually represents the different NoSQL systems.
For more information, be sure to check out the MongoDB - Humongous Drupal presentation from DrupalCon San Francisco.