I have just purchased a MBA and I have managed to get a shared partition working, although it took me a number of times to get it right.
As I have the Air, I followed the instructions here to get started with Windows on Mac without a DVD drive.
The standard Boot Camp installation only allows the hard drive to be split in two. After running the Boot Camp Assistant fire up Disk Utility, delete the newly created BOOTCAMP partition. Create 2 new partitions, with the 'middle' one being the shared partition. I used exFAT as the format, so I can have files over 4Gb. Use FAT32 for the 'end' partition, as the same as Boot Camp creates, although that will just get formatted as NTFS by the Windows installer anyway.
I also called the end partition 'BOOTCAMP', but I'm not sure if that is required. Apparently you need to make the last partition the Windows partition, although I haven't verified that either. Basically I have done this once and it works so I'm not going to format my drive again just to make sure :)
After that is done, restart and go into the Windows installer. From there select the 'last' partition (should be partition 4), format it with NTFS and then it should work as expected.