It's hard to say. I think it is a very bad fit for them - Sun makes money on hardware, not software, especially one that is mainly used for free anyway. If you look at history of big high tech acquisitions you will see that many of them, if not majority, don't really playout well at all. It is safe to blindly bet that a big buy like this will fail even without knowing the details - Ashton Tate, Netscape, ICQ, WinAmp etc.
Oracle is certainly not going to be happy, and it is really them who would pay the most to get rid of MySQL. The only way I can see it might make some sense for Sun is to resell MySQL to Oracle at some point - perhaps at the point when Sun itself will be broken up to pieces and sold to the highest bidder: Java to IBM, MySQL to Oracle, SPARC group generally closed with some people hired by Intel.
I have to say here well done to MySQL people - they certainly worked very hard and they deserved good payday. They also provided very good value to a lot of people without charging much for it, even though some of their recent actions are a bit questionable, but hey, they achieved great success with hard work, so I think they can be forgiven for some of the not so good things they did.