Upon further thought, I'd like to change (somewhat) my comments from earlier.
What I DO like about Arduino, and Raspberry Pi (I own an RPi for full disclosure) is that they do, indeed, get young people more interested in electronics. As my generation is now hardwired to want instant gratification, these provide a gateway as others have mentioned, to have something 'just work'. And yes, this can be a great way to get into more advanced stuff. I was browsing the SparkFun website the other day and I was actually thinking about trying to get an Arduino to play with. However I don't like the shields idea -- the whole point, at least to me, should be to provide a simple base to build off of. If you want more digital I/O, don't buy a Gertboard (for the RPi), wire your own!! You'll learn tons more doing it yourself than just buying a shield.
Particularly with elementary-aged children, I think RPi and Arduino could easily help solve a huge problem that's been growing as the 'disposable' generation matures. People throw away broken electronics because nothing is user-serviceable. That's why I love the concept of websites like iFixit, because it encourages people to try and fix it themselves (thus saving landfill space, and increasing recycling and reuse) and also saves you the money of buying a new whatever. Children LOVE electronics, and when you have a 'turnkey' solution like the RPi and Arduino which take like 10 minutes to set up a working project, the amazement and wonder in their faces is priceless. I'm going to be volunteering at a Youth Centre and I've been thinking about bringing some electronics along to try and stir up some interest. The kids will be from ages 11-17 if I remember correctly. Perfect age to get them interested in electronics!
Anyway, back to my main point. I believe that Arduinos can be good for the above reasons, when used as a 'jumping-off' point to build a project, or for quick prototyping. What I dislike is that they seem to become a crutch. As others have mentioned, people just stick with their Arduinos and shields, never progressing, simply following tutorials that tell you 'stick wire A in slot B' to get something working and never comprehending the basic, fundamental concepts that underpin all electronics. If these were used in their proper manner, I think it would spark interest to the point where people will end up devouring books on electronics, and wanting to solder their own. There's nothing wrong with 'dabbling' in electronics, buying an Arduino, and making an LED flash, getting bored and shelving it, but when I see kids making robots out of fully pre-assembled boards, and downloading the firmware already compiled, and simply copying it over -- that's what gets me upset. That's where I think Arduinos and the like are causing problems, rather than fixing them.
What needs to change is basically, people using these products need more guidance! I know personally that my interest was sparked as a child watching my dad fix electronics on his workbench. All the different tools, and wires, and soldering irons, intrigued me and that passion has never left me. Finding Dave's blog has simply re-sparked that passion and pushed me down the path I was going to take eventually anyway -- going to college for EE. However, I think that if all my electronics learning had involved pre-assembled boards and no reading/soldering, I wouldn't have been as interested and probably would have gotten bored and stopped tinkering.
Sorry for the long post, I'm just trying to articulate myself well.