I have a retina MacBook Pro, and I've noticed that several of the images in IPython need to be updated to higher resolution so that they don't look bad. What I've noticed so far:

• The icons in the notebook, including the reload icon on the dashboard and all the toolbar icons in the notebook itself
• The IPython icon in the header of the qtconsole
• The IPython icon itself from the qtconsole (Note that Mac OS X supports extremely high resolution application icons)
• The IPython icon in the quit screen in the qtconsole
• The default resolution for inline plots in both the qtconsole and the notebook is too low (this one is probably the most important).
• The favicon for the notebook.

Note that you'll need to use Safari to test the notebook, as Chrome doesn't fully support retina graphics yet (Chrome Canary supports retina graphics, but not retina favicons).

This is only based on my little knowledge. Likely there are images and such for features that I don't know about that also need to be updated.

On non-retina Macs, you can test this by using Quartz Debug to increase the scaling to 2x.

Sorry if this seems low priority, but you really have to use a retina device to see just how bad non-retina things look.