Yes, you want mp3gain track and album tags.

The mp3gain tags will not be read by any of the iPods. Apple use their own tag for storing gain data. When using iTunes this gets automatically calculated for each track when you enable Sound Check (I don't know whether it calculates track and album gains or just track ones).

I uses iTunes for syncing my music to my iPhone, so this isn't a problem I've had to deal with.

This script claims to be able to apply the Apple tag using the mp3gain tags as the source and so should allow iTunes free volume leveling:

http://www.vdberg.org/~richard/rg2sc.html