Quick update regarding the new MacBook Pro with Retina display. Curio's pens & brushes don't currently work with the retina resolution. Existing scribbles appear just fine but new scribbles aren't rendered correctly while drawing them, until you escape out of sketching mode.
This has been fixed in the upcoming Curio 8 as the internals of sketching have been rewritten so stay tuned for that release. We're working as quickly as possible on it -- Apple needs need slow down the curve balls (Mountain Lion, Retina, AppStore changes...). :-)