Actel devices are often used because they use flash memory for the configuration, which makes them usable in situations where RAM may be untrustworthy for something so important (e.g. where you can get bit-flipping due to ionising radiation).

They also make properly rad-hard FPGAs, although these are ridiculously expensive (I've heard that if you are looking at building more than ~10-15 units, an ASIC may actually be cheaper).

Edit: I should add that you could still use Xilinx and Altera devices in these situations, but you really need some way of mitigating the problem -- such as TMR memory.

Xilinx also has flash fpgas, the Spartan 3AN and I believe altera also should have one.

About the rad-hard cost, yes they are >10k u$s each in most models but rad-hard ICs also are very expensive, I doubt an IC will be cheaper unless you buy by the millions. Anyway if you need something to be rad-hard, cost is the least of your concerns.

ICs have their hardware 'configuration' effectively in the metal layers, which are not so susceptible. The configuration is the biggest concern, as this is essentially the processor (or whatever the circuit is) itself. RAM used by a processor is also at risk, but this is easier to handle. From what I'm told, a reasonably large feature size ASIC can have far fewer problems than a modern FPGA.

Also, note that the Spartan 3AN does not seem to use flash for configuration. It looks like the internal flash is just for storage and the configuration is copied into RAM, as with normal Spartan devices. This means one less chip on the board and potentially better security, but no real advantage for avoiding SEUs.