Nintendo of Europe updated their New Super Mario Bros. page and spilled details on “demo play.” Contrary to what was said in a Miyamoto interview, demo play won’t beat a level for you.
Here’s an excerpt from NOE’s product page. Note the bolded section, emphasized by us.
New Super Mario Bros. Wii is also the first Nintendo game to feature a dynamic help system, which allows you to access a mode showing how a level can be completed if you are stuck. The best thing about this mode is that you are free to jump into the action you’re watching on screen at any time!
So, instead of the game beating a level while you grab a snack, it shows you what moves to make and you can mimic what happens on the screen.