MySQL Clustering v1.0 ...
only works if you can put all your data in RAM, according to O'Reilly's Andy Oram. That's a big limitation.
OTOH, he also mentions that there are commercial alternatives for MySQL clustering that don't have this limitation, like Emic
http://www.emicnetworks.com/ - clustered MySQL
If you don't need a true RAC-like single-system image, but can deal with a replicated solution, you can do what Sabre did for their MySQL cluster and choose a commercial replication solution:
Also via Jeremy Z, very cool slides on LiveJournal's porgression from vanilla LAMP to fault-tolerant megasite:
I have to say, I love case studies like this. They're great ammo if you need to pitch an open source alternative, or maybe just keep your Oracle rep honest.