Reading comics on the new iPad's retina screen is about to get a lot better. New comics from Marvel, DC, Image and other major comic publishers are all available in retina-level resolution, and back issues and indie comics should follow suit very shortly.
The bulk of the retina comics comes from Comixology's freshly updated apps, which power Marvel, DC, Image and other publishers. About 125 issues are available in the "CMX-HD" format to begin, covering every issue released last week, and every issue released from this point on will also be in HD. Comixology and the publishers are also beginning work on making their entire back catalogues available in retina within a few months, hopefully.
Graphicly, the main competitor to Comixology in digital comics, has its retina app submitted and is just waiting on approval from Apple. It also offers Newsstand subscriptions for several comic titles that let publishers push new issues directly to users; those are already updated to provide HD downloads.
As for actually getting the new files, the Comixology apps will recognise if you're using a new iPad and automatically offer to re-download any already-purchased comics in the new HD format. Graphicly didn't say specifically how it would work, but expect something similar.
The CMX-HD format files are a little more than twice the size of current files. Graphicly has emphasised image quality over size all along, but you can expect a noticeable size bump from its stuff as well. Which could be rough on your iPad's storage -- especially since new iPad users won't be able to download non-HD format issues. But then, Comixology and Graphicly had both been sending files at higher-than-iPad-2 resolutions for a while, which meant there was a noticeable bump in image quality immediately on the new iPad already. So it's not like you had super small files to begin with. Retina resolution should improve the quality even further and eliminate pixelation when zooming in.
Comics already look really, really good on the new iPad, so we're pretty excited about digging into even purdier books from here on.
Update: Looks like DC's app isn't updated just yet, so Marvel's the only major publisher that's live with retina so far. We were told that DC and the other publishers would be up today, too, but we've reached out to try to see if something went wrong.
Here's a sample side-by-side of the old and new resolutions, zoomed in on a panel. Click for the full size.
And here's another from Marvel: