MySql is something of a Red Herring (passing through the Red Gate headed upstream to spawn) in the Oracle/Sun deal to my way of thinking, from the beginning.
Larry has a history. That history is an obsession with beating IBM to death. Oracle produced the first relational database, based on Dr. Codd's work, before IBM. The software acquisition binge has been to further that end. So is the Sun acquisition, but not to get java. java is already theirs, to the extent they want it.
The reason for the deal was the hardware stack, both as it is now, and what it can be. The latter is the key to understanding the deal. Historically, Oracle as run like dog poo on IBM mainframes. This is due to intimate knowledge of the internals of the (now) z/processor, and the contrary roots of Oracle as unix software.
With the Sun stack (especially with SSD now a integral part), Larry now has an offering to replace the IBM mainframe habitat. Since Oracle can't run on the IBM mainframe, turn the Sun iron into an Oracle mainframe. They've tried it with the HP database machine, with some success, but that required collaboration. Now Larry has a full playground in which to work.
MySql is not a useful steppingstone to Oracle. Postgres is (was and will be, too). Oracle is an MVCC database, while MySql and DB2 are traditional lockers. Unless InnoDB is written to MVCC semantics (how much work that requires, I don't know), MySql isn't an entry level Oracle. It's just another piece that's come along for the ride.