*edit* I was technically incorrect about max voltage supplied by USB ports, I\'ve changed my post to reflect the right information as provided by the r-pi team in this thread.
1. Yes and No, the USB ports are powered (they receive the power straight from the PSU), but at this time there\'s a fuse that limits the available voltage to the ports to 100 mA. If you replaced the fuse with a higher amp one then since the 5V \"rail\" is connected directly to the USB ports then the simple answer is the USB ports will provide as much power as your PSU can handle though I would be wary of going above the usb 2.0 standard of 500 mA per port).
2. The .7Amp figure I\'ve been mentioning is coming off this post
on the front page. At the time of this post it\'s the second post on the front page, but the gist of the post is that .7 Amps is a theoretical max draw based on max load of the board itself and a high powered peripheral attached to it as well.
*EDIT*: I made a mistake here saying a Eben estimated 300 mA for model B, the front page says 300 mA for model A
. I\'ve corrected the mistake in this post to avoid confusion. */EDIT*
It\'s been speculated (by Eben) that 300 mA would suffice to run a model A. As you can imagine for the Model B, however, two USB ports supplying 500 mA and an additional 300 for just the board means that you\'d actually need a 1.5 to 2 Amp PSU if you wanted to \"max out\" the USB ports, but in the event that the ports ask for more than the PSU has to offer, Liz has already told us the the r-pi is smart enough to not provide more than the PSU has and the device will not receive power.
This is where the powered vs unpowered really comes into play. I\'ve yet to meet a USB port on a computer that was not powered off the computer\'s PSU, but a USB *hub* can be unpowered (relies on the computer\'s USB port to power any devices attached to it) or powered (has it\'s own PSU to provide power to its USB ports). *IF* you need more power than the max provided via the USB ports then you\'ll have to get a powered USB hub. But then that raises the question, if you\'d have to buy a powered usb hub anyways, why not just get a hard drive with its own PSU or even its own wall wart usb adapter?
To answer the rest of question two (Will it vary between the two initial models) the answer should be only slightly provided that there is nothing attached to the boards
The only major differences in power *should* be based on what you have attached to the USB ports and maybe the ethernet port?.
3. Just like any other electronic device, when there\'s an undervoltage, the r-pi will turn off
I also don\'t know the tolerances of the fuse, but a 1% tolerance on a fuse would be pretty ridiculous IMHO, I think the tolerance will be strikingly similar to the tolerance found with cell phone chargers. (We are dealing with a mobile