Thank you all for answering!
So to summerize, if I run 12V into the barrel jack I can take 1 amp from the VIN pin and 800 mAh from the 5V pin.
For a very limited period of time. You are hitting the edge of the engineering tolerances, and while it will probably technically work, it is not a good solution. CrossRoads brings up the very good point that the more voltage you drop across the linear regulator, the faster it heats up. It 'eliminates' the extra voltage by literally burning it off as heat. So regulator needs to burn off (VIn-5)*(Current) Watts, which can quickly heat it up. So while it works, it is not a good idea for personal safety and lifetime of your circuit. And while it claims 800mA, I honestly doubt it can sustain that for long. Finally, if you are doing anything with a battery, you are basically wasting power heating something up.
Summary: Minimize large current applications drawing off of the Arduino board. My preference is about max 200mA, but I cannot give any concrete justification for that particular number. I would, if at all possible, have the Arduino board power as little as possible.