Separated at Cloning: 2010 Honda Insight and Toyota Prius
On the left, the new Honda Insight, as snapped by CrunchGear, with a (surprisingly low) claimed 43MPG fuel economy rating; on the right, the right, the new Toyota Prius, given an eye by Top Gear, with a estimated 50MPG fuel economy rating.
The Honda will be a hair less expensive, but the Prius is certainly still a contender. A larger engine, even lower coefficient of drag (at .25, the lowest of any mass market vehicle), and a solar-powered air conditioning system make the Prius a bit more upscale.
More looks:
• Up Close: 2010 Toyota Prius [Kicking Tires]
• 2010 Toyota Prius: Bigger Size Meets Bigger Fuel Economy - 50 MPG! [Jalopnik]
• Driving the 2010 Honda Insight [CNET/Cartech]

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Is this simply a matter of convergent evolution, or are they being built (relatively) interchangeably (i.e. platform sharing) at the same assembly plants?
Convergent evolution: There's really only so many ways to get the drag down. Plus, I suspect Honda won't be brokenhearted to have people confuse the Insight for the Prius.
That "hair" less expensive is currently estimated at $3,500 (18,500 vs 22,000) which is a bit over 15% less than the Prius. I think that's pretty significant.
The Honda will probably be a lot more enjoyable to drive, too... compare Civic to Corolla, Accord to Camry. Not that it will be really fun to drive, but a bit less numb.
I just wish they didn't look like door stops.