I want to mount my Uno in a nice Aluminum enclosure, with the DC jack and USB port sticking through the rear panel. I will power it with a 12VDC wall wart.
My question is, I'm needing 12VDC on some other circuitry inside my enclosure (to run a sollenoid), so is there any reason why I can't carefully solder a wire to to back of the DC Jack on the Uno, to tap off the 12V coming in? (There's no real place on the board to get this power, is there?)thanks
Yes there is a place. When the board is being powered by the external power connector, that voltage is then available on the Vin pin of the arduino power shield connector to be used for external components rated to use that voltage, +12vdc in your case.
However the first thing you have to determine is what the current demand for your solenoid is? There is a series polarity protection diode between the external power connectors positive terminal and the Vin pin and it probably has a 1 amp maximum current rating. Your external power module also has to have a maximum current rating sufficient to power the arduino board and all other external components being powered.