The command line is simple.For certain definitions of "simple" :-)
I'm one of the people who learned of a simple command line (although on a Sinclair ZX Spectrum - didn't have access to nice little unix boxes in my day).
And of course it's a fine and good way to learn how to program.
That said - I've seen a few teens take a different route. They got into MySpace. They started wanted to "tweak" things. That got them into HTML/CSS. That got them into JavaScript. That got them into coding.
Or I've seen people start with hypercard back when hypercard was the biz.
Or I've seen people start with Director (and later Flash) and move on from there.
The other thing I saw several times back in the day when I taught development to non-dev folk was that some folk were really put off by the command line. I remember we had a course that I taught that was built around an GUI IDEish environment where folk were building a little SHRDLU type app. At the same time a subset of the course were on a "beginners C" course that was all command line driven. The C course was good and well taught - but some folk hated it, but made superb progress on the IDEish course. It was freaky to see the same people spend hours ranting about finding files and overstepping array bounds by one on one course, while successfully building basic NLP apps on another :-)
Now - I like the RasPi. It's a cute box. But I'm unconvinced that it's going to make any significant difference in the number or kind of young folk who go play with development tools. I can't really picture anybody (child or parent) purchasing one of these who wouldn't already be the type to poke at a home Linux box or PC in a developerish way.
With the way that cheap PCs get recycled is the price point going to be enough? It's great for hobbyist hackers - but they were already hobbyist hackers :-)