When I got home this evening and read my email from Capcom's PR dept., I nearly exploded with glee.
By the way, thanks for reading my Runner 2 review. Here's my reply I posted on the GamerDad.com site. I'll put it here to save you the trouble of having to go back there.
There are hidden retro levels in some of the stages if you find a hidden Famicom-shaped cartridge in that level. Then you'll be whisked to a retro-themed stage. After you find a retro level in a stage, you can access it any time you want to on the map screen. So if you have found a retro level on stage 1-4, for instance, when you are on that space on the map, you can press the 'retro button' to access that level.