Refined HD
Anonomys

Refined HD is designed by Anonomys, and features a very complete and polished theme, optimized for the Retina displays in our iPhone 4 (Hopefully the iPod Touch 4G too). This is one of David's first themes, and clearly, the MMi community has welcomed him warmly by nominating his theme as one of the best iPhone 4 themes on ModMyi this very moment.
The Theme features a clean, and crisp UI, all while sporting the wonderful Masking techniques used to auto-create icons to match the theme without the need of a huge iconset.
More information can be found in the forums.
iCatalyst HD
subywrex

Inspired by an MMi community favorite, Zausser, subywrex presents the community with iCatalyst HD for the incredibly sharp Retina display (and soon on "non retina" displays)
While still considered a WIP, the theme is an Open Beta release and you can participate with a small donation of your choosing.
More information can be found in the forums.
Stella
Chaitanya

This one isn't an entire theme, but rather a great add-on to almost any theme. Created by Chaitanya over at MacThemes.net, comes a sleek, sharp looking slider theme.
Download for iOS 4.x | iOS 3.x
Message