It seems that Nero NRG format is a fairly trivial overlay about the ISO9660 CD format. The easiest way to convert an NRG to an ISO is to strip the first 307200 bytes; the rest will be your resulting ISO-9660 image.

For example; to mount an NRG in linux via loopback: mount -t iso9660 -o loop,offset=307200 image.nrg /mnt/image

Alternatively, a program exists, which will do this for you. I’ve pre-compiled a MacOS X 10.3.x version. You may download it here.

[Edit: Added link to precompiled build.]