I tried to do this, but couldn't get it to work.
You can't repartition the drive and use the empty space. I've tried allsorts and nothing will format the partition in the format that will allow OSx to install.
You have to reformat the drive, via the OSx install and then repartition it from there.
You then need to reinstall windows 7 onto the second partition.
What you *might* be able to do is to repartition the drive, then take a Ghost image of Windows 7. Then format and partition again via OSx and put the ghost image back onto the now empty partition. Note you have to partition before taking a Ghost image or Ghost will want a bigger drive to restore the image.
This will mean your Windows 7 install is identical to what it is now. Ive found when reinstalling Win7 from scratch, you don't get all of the nifty features that come preinstalled. I started to do this, but in the end didn't restore the image and just installed Win7 from the original CD instead.
If Ghosting is too much hassle, or you don't know what I'm talking about with Ghost, then go with the standard guide on dual installation. You'll need your original Windows 7 CD. Loads of guides on here on how to do the dual installations.