Nvidia will still be 'in the game' for quite some time. The only question is how competitive they'll be.
Getting the 5000 series cards out was very well planned and executed; not only being able to get decent yields on TSMC's troublesome 40nm process, but also getting the entire lineup out in a relatively short time.
Anyway, the 6000 series is kind of a stopgap. It uses the same shader processors in the 5000 series, but uses the off-core logic of the next generation GPU's.
The reason Southern Islands even exists is because TSMC are having trouble getting their 28nm process working. It's something they're doing 'in the meantime' while they're stuck on 40nm.
Who knows, maybe they'll choose to use GlobalFoundries instead for Northern Islands?
You might consider it worth it to wait for NI and skip SI if you already have a HD 5000 series card.