Ok, I have to amend my previous comments that the HD sharing on the Airport Extreme works. While it does technically share the drives, it is effectively useless.
If you copy any more than the smallest files to the AE, it just doesn't work right. Last night I tried to copy a large amount of data (50GB) to the shared HD. It copies VERY slow. I was seeing about 1-3MB/sec. This is on a direct, wired connection to the router! At that rate, 50GB will take forever to copy. Plus, while this copy is going on, Internet access through the AE, even if it's just web browsing is horribly slow. To make matters worse, every now and then, the copy will just fail in mid copy.
All this testing was done using Windows Vista. Whether this problem is exclusive to Windows, I'm not sure. I'll have to try this on the Mac to see if it also happens. Irregardless, this is unacceptable and Apple needs to fix this immediately