As I understand it, the 'extra space' is invisible to the user. Basically, advanced format drives are able to cram more data into the same physical platter surface area (4x as much, if I understand it correctly). This means that they can start manufacturing drives with much higher capacities without increasing the number of platters. This is good for power savings, weight, heat, etc. A 2 TB advanced format HDD will still appear as a 2 TB hard drive to the user even when used in a supporting OS, such as Win7. There's no hidden space to be unlocked, or anything like that.
I agree that this whole issue is quite annoying, and it makes choosing a 2 TB drive rather daunting...they all seem to have some problems (at least all the green ones do). Still, pretty much every time a new technology is introduced there will be an awkward transitional phase. As consumers we just get to whine a lot and ultimately tough it out. And won't it be fun when all our non-advanced format drives (which I'm sure they'll soon start calling 'legacy drives' or similar) will be incompatible with new systems! Not for any good reason, of course, just because they want us to upgrade to the latest and greatest. Kind of like how Windows XP is now claimed by Microsoft to be 'incompatible with the internet' because it still ships with IE6.