In Debian Lenny (stable as of 2009) this file contains mappings for ethernet drivers that will be persistent across boots:
/etc/udev/rules.d/70-persistent-net.rules
If you change nics (netgear natsemi for an intel e100) for example, you will need to edit this file and remove the stanza (1 comment and 1device line that begins with SUBSYSTEM=="net",....)
Reboot, and the new nic should be detected as eth0