Stealth should decrease with light radius, because announcing your presence to the entire room with a huge light radius isn't terribly stealthy. Consequently, rogues & vampires and etc. should be given an infravision boost, which should be tweaked so that it is more useful. If the idea is based around thermal imaging (and I believe it is), it should be easy to distinguish between walls, map features, items and monsters with IR alone. i.e. http://en.wikipedia.org/wiki/Thermal_imaging Disable the effect when the player is wielding a light source, but at high infravision values, it should be nearly as good as having the level lit.
Likewise, Invisible players should be granted a stealth boost. There's something obviously self-defeating about those magi lamps which grant invisibility, but they would still be useful for their oddness and other properties. This would help to make invisibility more useful at higher levels, I suspect, because it would be interesting if it was harder for a monster with see_invis to see an invisible player than it is for them to see a visible one (it might be currently, I haven't checked for the purpose of this speculation).