Wouldn't it make sense to connect Vin and the V+ of the barrel connector before the diode?
This would give the Vin pin protection against incorrect polarisation (ie: connecting + to GND and - to Vin). As an added benefit, you can use the barrel as a power output if you use Vin to power the Arduino.
The only problem I see with this, is not everyone uses Vin as an input. Some (many) use it as a source of unregulated power output for power hungry items (motors, solenoids, radio transceivers, ...), I can imagine at least a few shields do.
If you're using Vin as an output, the protection/idiot diode is a nice feature to have. Going forward, a potential solution is to duplicate the diode on the attached object.
There is another advantage with the current position of Vin.
There are two ways to power the board from a battery : using the power-plug or going into Vin directly.
If you use the power-plug, the board is protected (by the diode) from hooking up the battery with reversed polarity which would kill the arduino.
But the diode will also eat up ~0.6V of the battery-supply voltage, thereby shortening the time the arduino will run. (The voltage-regulator needs a minimum input-voltage).
When you plug the battery into Vin directly, it will kill the board if you get the polarity wrong, but if you do it right the arduino will run significantly longer...
Thanks. I especially did not realize there was that ~0.6 volt dissipation in the diode.