It seems that putting a motor into a trailer and bolting it to the back of a bike would be a bad idea, giving an unstable ride and leaving the trailer itself jumping and skipping across the asphalt. But what do I know? Here's a clip of the Ridekick trailer in action, where it acquits itself admirably:
The Ridekick hitches to the back of the bike and shoves it forward via the rear axle, so it should feel somewhat natural to use. The 500-watt motor takes six hours to charge and will carry you for up to 15 miles (longer if you pedal some of the way). The whole shebang weighs in at 40 pounds (18 kilos) empty.
But the best part about having a motor in a trailer is that you now have... a trailer. Inside it has 42 liters of space to carry up to 34 kilos (75 pounds) of anything you like (we suggest beer). Not only is a trailer better than a backpack in terms of comfort, this one even pulls itself.
What I'd really like to see is a larger flatbed version, for hauling some more serious loads. I have no interest in motorizing my bike (how would I burn off all those beer calories?), but a big motorized trailer could be seriously useful to help keep up with traffic when carrying a load.
The Ridekick is available now, for a reasonable but not cheap $700.
Ridekick product page [Ridekick via Oh Gizmo!]
