You don't think Sun wants Oracle's slice of the pie and is perfectly reasonable for wanting it? They know better than anybody what that slice is worth...Sun and Oracle were the basis of most of the first Internet boom and both made out like bandits. Even if they can only make a tenth what Oracle makes, it's still worth every penny.
It would have been smarter to buy MySQL AB earlier...but Sun only recently really got that Open Source religion, and earlier they would have been reviled for stepping in and buying up MySQL (if the MySQL guys would have even sold to a pre-OSS friendly Sun). Sun also had a bad habit of buying and killing companies, which is presumably now corrected. The OpenOffice acquisition turned out well (ignoring the Java dependency issue already mentioned), so everybody probably trusts them to do the right thing by MySQL.
I think it's a strategic move for the company: MySQL might help positionate better all Sun products, and so they buy. I don't think they are going to get that kind of profit from that product alone, that's not the objective afaik.
Please don't get me wrong, It is Sun's money to do with as they wish, I am just trying to get my head around a billion dollars.
I doubt if they'll ever see a commensurate return on that investment. They'd have to make millions of dollars in profit per year just to keep up with the time value of money.