dc42:
One thing to watch for is that Rds(on) is often quoted at less than the maximum rated drain current. Always look at the datasheet and check that Rds(on) is quoted at or above the maximum drain current that you want to switch, for Vgs = 5V or lower.
Rds(on) doesn't change much with current if the device if Vgs >> Vds and you can model it as a simple resistance. Furthermore you won't normally go anywhere near the max current rating in a sensible circuit 
For instance a 0.01 ohm device at 50A will have Vds = 0.5V - if Vgs = 10V then Vgd = 9.5V and the channel is basically the same geometry as it is at zero current (where Vgd = 10V). It will be dissipating 25W too which means v. good heat sinking needed. For
50A I'd be tempted to go for lower Rds(on) or multiple FETs - thus reducing Vds.
So I'd reword this as "calculate Vds(on) and check its a lot less than Vgs(on)"