Swap files in Linux are stored in a special filesystem called "swap" which stripes the data across volumes. It is not encrypted.
It can be made to be encrypted. I only know of one majordistribution that has this feature (Mandrake) and it isn't well known nor enabled-by-default. The reason is that Linux is mostly used in a server environment where it is hard to physically compromise the machine.
There are a lot of "secure" distribution versions of popular distributions (Knoppix-MIB for instance) and software add ons (via init scripts) that can enable it. For the most part they work by doing what I suggested earlier with the minor difference that swapfiles in Darwin are files, not filesystems.
They also can be made to encrypt or hold resident in RAM /tmp. Another nice feature.
The best solution in the Darwin world is to port the secure swap features from OpenBSD.