I don't know. The HiDPI modes already seem pretty usable to me. And while current resolution artwork doesn't look sharp on a 'retina' display, it shouldn't look any worse than it does on the low resolution displays we're used to.
EDIT: Actually...Q2 2012 seems way too early to me, too.
EDIT2: Icon previews are going to really look beautiful (and be even more useful)...:
Image
Besides application icons, what DOESN'T have a HiDpi mode?