In a e-mail announcement sourceforge says they'll be switching to IBM's database DB2. As an icon of open source computing it seems a bit of an odd move to go to a closed proprietary platform, at least from an optics point of view don't you think?
Looks like the deal has a lot more to do with making money than practicality. SF is owned by VA Linux, and they are seeking a business relationship with IBM to resell DB2 and WebSphere. IMO it kind of stinks but I guess it is better than many alternatives.
Maybe they are getting to large and MySQL was not holding up as well anymore. I know I have read that MySQL can not hold up against some of the big boys in massive DBs.