I found that nrg4iso (on the Mac) converted .nrg to an .iso file that, when burned to a DVD using Disk Utility, was playable on my standalone DVD player.
nrg4iso can be downloaded from http://code.google.com/p/nrg4iso/downloads/list (it's the Mac OS X executable). It's pretty straightforward. You run it in Terminal and for some reason you have to type "./nrg4iso". So, for example, you would type "./nrg4iso foo.nrg foo.iso" to create foo.iso from foo.nrg. I think that if you type the command according to the example you have to be in the directory that contains both the executable and the source file.
The reason I did this, incidentally, was that I wanted to author the DVD using Nero Vision on my PC, but burn it on a Mac, because I don't have a DVD burner on the PC. I created the .nrg file in Nero Vision, burning to Image Recorder with the Best Quality setting. I was happier with the results than with other methods I tried.