I was curious on this, because around the interwebz, that is something that is highly debated.
Now, there is no right or wrong in this discussion. I'm not trying to get a definitive answer. Just seeing what everyone's thoughts are here.
Personally, I think the retro cut-off is when retail outlets (that don't sell used) stop carrying the system.
So I'd personally say that PS2, XBox, GameCube, and GBA-SP are the most recent retro consoles. (Yeah, I know, the DS, PSP, and Wiis are getting harder to find in retail outlets. But you can still find them.)