I wanted a three partition Boot Camp setup: one OS X partition, one NTFS data partition, and one Windows 7 partition. After trying some of the other comments, I found a very easy way to accomplish this:
1. Run Boot Camp assistant and install Windows 7 as per the installation and setup guide.
2. Purchase Coriolis System's iPartition software (about $36). Upon downloading it, you are directed to create a bootable CD. Boot with this CD, and use iPartition to resize the Macintosh HD and Bootcamp partitions to whatever size you want, and create a new partition for Data with whatever format you want (NTFS in my case).
3. Reboot and run Verify Disk Permissions and Verify Disk, just to make sure everything is OK.

Yes, iPartition costs money, but this is one of those times where EASY beats FREE...

The Bootcamp and Data partitions show up as disks and can be read by OS X. For read/write access, you can purchase and install Paragon Software's NTFS for Mac on the Macintosh HD, and HPS+ for Windows in Windows. Then all disks are fully accessible to both operating systems.

I also suggest immediately making sure Time Machine has a fresh backup, and getting WinClone to make an image of the Windows partition, as a further backup. If there is any problem, you can delete the Windows partition, recreate it, and restore it from WinClone. [In my experiments before finding this solution, Restoring Windows from WinClone was so much easier than doing a re-installation of Windows!]