I did some research on MySQL cluster so i can recommend one of our customers whether to use it or not. It turns out that the MySQL cluster is very different from other DBMS clustering solutions in that it distributes the data across the nodes (synchronously) and then stores it on the disk (asynchronously). This is all very nice, but at the moment all data is stored in memory, so if I have a 120GB database I would need around 150GB memory on each node, which is silly.
The Wikipedia Page on MySQL Cluster gives more details.
Waiting for MySQL to get better...
UPDATE: It seems that this is no longer the case for the 5.1 version of mysql. See here.