Now that WordPress MU will be integrated into the WordPress code base, it would be nice to see Cassandra integration. This be huge for very large scale multi-site deployments of wordpress.
With very large scale MU deployments, it puts a huge burden on MySQL and DBAs to backup, replicate, and maintain your 10,000+ MySQL tables. Cassandra is relatively much easier to maintain and it has no single point of failure.
Of course, such a backend change is definitely not for everyone. Certain types of queries that a traditional RDMS like MySQL provides will not be possible with Cassandra; thus, prevent the migration over to Cassandra all together.