ethX is the kernel-assigned namespace. All the initial device naming happens in parallel by the kernel, with a lock on the bits that hand out the next available integer value. Renaming then also happens in parallel, and they all race. The simple swizzle of eth0eth1 through rename scripts can leave a system with both eth0 and eth0_rename -named devices, not what you had in mind. The only way to avoid the races is to move out of the ethX namespace and into another. I wish that weren't so - this bit is what has kept this feature from happening for 4 years - I didn't want to change namespaces. In the end, I had no choice.

@lordholm: the initial assignment is non-deterministic; not quite random, but not necessarily the order you assume. On many systems, the on-board NICs are discovered after some PCI slots are scanned, on others, the on-boards are scanned first.