The answer is "elsewhere and nowhere".
The vast majority are still at Oracle, some have left, but plenty are still there. This got me thinking about "who wrote what". Innodb is a sizable piece of code and it continues to be at Oracle. Without Innodb, you don't really have a database that is 24/7. Innodb has been there for years.
What about the other pieces?
The optimizer guys are almost gone. They left a while ago to MontyAB.
Replication? They are all there minus the guy who wrote the first version (which is still pretty much what is used today). Him? He does consulting for different companies, but he left MySQL in 2002 or so. The execution/trigger system is a bit of combination. The "pre-stored procedures" stuff was all written back in the day, and little of it has changed, everything past that? That work was done by folks who are still at Oracle.
The parser? There is not much to the MySQL parser. A large chunk of it came from the merging of MiniSQL and Unireg (which is the only big thing I would say is wrong in Henrik's Map of MySQL Forks). Most of the additional work done there was for the monolithic stored procedure design that went into 5.0, and later stuff done by partitioning.
The guys who wrote the connectors are all at Sun still. I've not heard of a single one of them leaving.
The person who wrote stored procedures? He decided to go fishing back in 2005, I don't believe anyone has heard of him sense.
So the people who make up MySQL? Most are at Oracle.
How about the "group leaders" that made up the early MySQL leadership? Three are with MontyAB, another went fishing, and I work on Drizzle.
How about Drizzle? There are a few folks who did early patches on Drizzle that are still at Oracle (and I don't believe they are allowed to contribute any longer). Five are at Rackspace, one is at a storage engine company, and I am happily taking contracts on engineering work and spending the rest of my time playing with my aging dog. There are a couple more Drizzle folks who I suspect by the end of the year will be core developers and I am curious to see where they land (keep in mind, since our creation we passed 150+ contributors well over a year ago, so we have people all over the place).