I'm not sure there actually is. All those small legal texts in software updates... Also: How to proof that it was actually one of their software updates that killed the slot? They could claim it was 3rd party RAM or bad handling of the PB or anything, basically... Until they actually have a system in place (free motherboard swapping for owners of PBs with serial numbers XYZ), there's not much a private person can do. I could go yelling at staff at the store, but they're not even Apple's people, they're just store staff (no Apple Stores in Switzerland...). And that wouldn't do much good.
Currently, my PB runs okay, so that's that. I'm not a big fan of the way it's been handled, and I certainly would be happier if it hadn't happened to me, but then again I was "lucky" with so many problems other people had with their machines and software updates (iTunes 2.0.2 erasing harddrives, Panther final erasing FW drives...) that I guess I'll just accept this one for now.