The udev rules change with the card and the drivers.
I'm guessing your original card was onboard? As the only aetheros wireless nic, it got ath0 - when you added the pcie card, you had two and the setup had to reconfigure.
Hand-wavey explanation follows:
You'd expect a new one to arrive at ath1.
But the situation can be more complicated than that. If adding a pcie interface card changed the way the bios represented the onboard unit to the kernel (changing where it is on the bus for eg) then the kernel, and udev, will register two new cards instead of one.
ath0 is already taken.
BIOS usually puts pci(e) cards ahead of the on board cards on the reasonable assumption that if you have a duplicate, you want the new one to replace the old one.
ath1 would be the next place - but we see it didn't - perhaps the driver prefers odd-number interfaces (maybe the odd-numbers share an interrupt that the driver uses? We used to get this with COM ports.)
Anyway, this would put the available devices as 0, 2, 4 etc. 0 is taken. The next ones are 2 and 4, which you got.
When you install the driver (+ card) the system writes udev rules to cover the configuration. Usually following the persistent-net-generator.rules as mentioned. But not always.
This is guesswork though - without knowing the bios and the driver in more detail I cannot tell what is really going on. But it's often something like what I've said above - and it should give you an idea how complex the OS has become.
It is probably OK to change one interface back to 0 by editing the udev file - though it will be interesting to see the effects. But if you just want to use the new card, it may work better to disable the old one in bios.
If you really want to know the details, you'll want to ask on the driver mailing list.
Last edited by Simon Bridge; 03-27-2009 at 08:51 PM.