It's happening... Yes, the laws are a bit funky, but reasonable for little motor vehicles that you can't ride on non-motorized bike trails (though people do anyway and I haven't seen anyone complain) but aren't quite full motorcycles (but can almost be used that way).
I'm starting to see a fair number of these tooling around: http://www.radpowerbikes.com/p... (granted, I live near Seattle where they're based) and they're pretty much the sweet spot. I'd get one if I didn't live near at the confluence of so many non-motorized bike trails (all of the old rail and trolley trails are being converted to bike trails in all of the major metro areas I've ever lived).
In other areas and downtown Seattle, these things are popping up more often, though. Lots of couriers use them to make deliveries on downtown bike lanes, weaving comfortably through the gridlocked traffic and purring up and down the hills. It's also big enough to take an adult passenger, and I've seen a woman take a bike like this to the library with 3 toddlers in childseats.
750W (1HP) is the limit for these electric assist motors in most states, (WA actually allows 1kW before classifying it as a motorcycle). There's also a grey area where the motor assist should be limited to 20mph, but it's fine if you push it faster by pedaling/going downhill. These kind of motor systems can be bought direct from China for under $300 ... and then another $300 for a pack of Li-Po batteries, so you could convert any sturdy bike into your own pedelec. The best hack I've seen involved building a bike battery pack out of ~$100 of surplus laptop battery packs from ebay.
I'm glad this is taking off, I've always dreamed of building a little 3-wheel velomobile as a kid, and all this stuff is going to make it much more affordable. We already use our cheap normal utility bikes as a second car, and it would be neat to have some electric options in the fleet for certain errands or to entertain visitors.