They become enlightened while they are still here.
There's a absolute quality to enlightenment, in that once it is attained it is,
1. retroactive, in that you can see that it was there for you all the time, and
2. ubiquitous, in that you can see that all beings have the enlightened essence already, just that it is covered temporarily. So a bodhisattva sees all beings as already enlightened.
The way I've written it here it's a cop-out I know, but there is profundity to it that isn't represented by my post. But that's just my opinion, not backed by scripture.
A human being has his limits. And thus, in every conceivable way, with every possible means, he tries to make the teaching enter into his own limits. ChNN