What is the best RDP/VNC client for the new iPad that makes use of the retina display?
Screens by edovia supports retina resolution on the iPad from day one for the iPad and works well on iPod touch, iPad and iPhone for quite some time.

Here are pictures showing the same desktop in retina and then non-retina iPad screen captures.

Retina:




Non-Retina:




Oddly, when you zoom in to the maximum zoom level (and you will likely need to expand these pictures if you are not viewing them on a retina display or they get down sampled to render on this site), the non retina iPad anti-aliases text a bit better than the retina iPad where you can really see the anti aliasing. This is similar to scanning print magazines where a lower resolution is often better for digitizing the "human perception" and not the "reality" of halftone screen details in process color printing.