Swapping over NFS requires a patch (http://www.instmath.rwth-aachen.de/~heine/nfs-swap/), but is not stable (yet). It is also very slow and insecure (memory pages go unencrypted over the network).

Swapping over NBD (network block device) seems to be buggy too; BasVanSisseren reported that swap over NBD crashes after some time (it looked like the client swapped out itself).