What happens?

It works. It's one of his skill's many strategies.

Rape happens. seriously zilean and evelynn laning together is the best combo ever. Have zilean place a bomb on eve's head while shes invis then ravage from behind just before the bomb goes off.

Any AoE ability will hit a stealthed character. So will skillshots.
A Cho'Gath can roar a stealthed-low-hp champion for the kill.
I've assasinated someone as twitch, stealthed, started to walk away, and been killed by an ECA.

As for placing it on an ally, it works. They also can't see the bomb.

If a bomb is on a champion, but nobody can see it, does it still do mass deeps?

Shen and Eve/Twich/whatever works well too.

Zilean is a hard counter to stealth champs, why?
placing a bomb leaves them unable to stealth until it detonates (3,2,1, damage :O)

That is if they're on opposing teams :P

He means what happens if you put it on your team mates and they stealth + go up to a champion. Mass dmgz? I don't know. Have to try in practice now that its back up ^^.

It works and can really give you a good laning phase, the constant barrage of bombs will wither their hp, and they need to decide to take it or go out of exp range.

Until they buy a ward.