MySQL Cluster RAM requirement From MySQL 5.1, the data doesn't need to be entirely in memory anymore.

I've read that the indexed columns (I think the entire index structure) must still be in memory (MySQL High Availability, 2010, pg. 533, "MySQL Cluster keeps all indexed columns in main memory").

In light of this, what happens if there's NOT enough memory (ie, a huge database (> 100GB, or 1TB), running on servers with low memory configs (2 data nodes, each with 1GB of RAM, for instance))?
MySQL Cluster supports storing non-indexed columns on disk-only with an LRU cache of recently accessed data.  However indexed columns are always held in-memory.  

MySQL Cluster preallocates all memory, according to the DataMemory and IndexMemory parameters.  It will not ask the underlying OS for more memory dynamically.

This means that you need to have configured enough memory across your cluster to hold all indexed columns in memory.  If your dataset is large enough that the indexed columns are larger than the available cluster memory, then you cannot load that dataset into the cluster.  At some point you will run out of space, and your inserting transactions will be aborted.

When configuring DataMemory and IndexMemory, it is best to limit yourself to somewhat less than the physical memory in each system.  Some physical memory should be reserved for the OS and other processes.

Theoretically MySQL Cluster can be configured so that it uses virtual memory via a swap device (e.g. more than physical memory), but as the other answers state, this is not a designed-for use case.  Having in-memory structures swapped to disk is usually sub-optimal as in-memory random access patterns result in random access to the disk, resulting in swap thrashing and slowdown across the system.  With MySQL Cluster, the most likely outcome is heartbeat failure and cluster failure due to a swapping data node not responding to signals quickly enough.  

To efficiently support larger-than-aggregate-memory indexes, MySQL Cluster would need to support on-disk index formats (perhaps a B tree etc) with caching and access patterns aligned with disk access properties.