As most of you know, Sun acquired MySQL (and later Oracle acquired Sun), and during these acquisitions, there were a lot of FUD in MySQL community which resulted in creation of
All of the 'forks' you mentioned (except Drizzle) re-base off newer official releases of MySQL. I think that the word fork can lead you down the wrong path - since the intention is just to provide after market modifications. I wrote about this here:
http://mtocker.livejournal.com/50931.html
Since they re-base of MySQL, and MySQL 5.0 is in "extended maintenance", only major vulnerabilities are to be fixed. This means you want to be moving to 5.1. If we work with this assumption, then it cuts OurDelta out from your decisions - since they are just the 5.1 MariaDB build/packaging partners.
I think you can also rule out Drizzle, since it is currently pre-beta. They're planning a beta by the end of the summer - but it's still much earlier than what you'll want. I really don't think you use a database not ready for production, so this rules out MySQL 5.5 as well.
So this leaves Percona Server, Official MySQL 5.1 and MariaDB. It's true that 5.1 has some poor scalability with many cores, but if you enable the InnoDB plugin it is better.
The rest of my response from here on in is biased - I work for Percona:
Percona XtraDB has additional CPU scalability fixes than MySQL 5.1+InnoDB plugin. The ones that are going to matter are covered here:
http://www.percona.com/docs/wiki/percona-xtradb:features:start
One I can comment on in particular that will matter is this one: http://www.percona.com/docs/wiki/percona-xtradb:patch:innodb_split_buf_pool_mutex