Beastlykings,
The power supply is rated to deliver 1 Amp of current (not Amp-hour) at 12 Volts. This does not mean that the Chumby will actually require this much power to run, only that it could supply that much if needed. I am sure the power consumption of the chumby depends on what you are doing. For example if the screen is on full brightness and you are pumping out max volume on some music from an attached USB drive or IPOD the current draw will be much higher than if it is sitting there in night mode.
With regards to batteries, they are rated in amp-hours. So your numbers for a typical 9V battery mean that it can supply about 500 milliamps for one hour. If your device is only drawing 100 milliamps, it will last 5 hours or only one hour if you did draw 500 miliamps. Keep in mind that by the time you are at the hour mark, the battery is going to be completely dead. So your chumby or other device is most likely going to shut off well before you hit the hour mark due to the fact that the voltage level will have dipped below the required level.
Now for your six AA rechargeable batteries. Since you are using them in series to get up to 7.2 V (6 x 1.2) you will only get 2500mAh total capacity as each battery is only supplying 1.2V at that capacity. You do not need to be concerned about having too much capacity, the more the better. The higher capacity just means that it can supply the same ammount of current for a longer time. The only thing here is that I would recommend to anyone making up a battery pack is to put an inline fuse in the power cord. This way if something goes wrong you will not have a fire or other similiar problem. As you get to higher and higher capacities, these batteries can supply a large ammount of current in a short circuit condition. A fuse is an easy safety measure that will help prevent a hazardous condition.
I hope I have answered some of your questions, if you need more info you can pm me.
-HuckFinn