Try this, it worked for me:
1) In your existing Snow Leopard installation, launch Disk Utility.
2) In the Partition tab, there is a block diagram of your Hard Disk, which will presumably show as one great big partition.
3) Grab the bottom right hand corner of this, and you will be able to resize the partition. Don't worry, this is non-destructive (although it's always a good idea to have a back up, yeah?)
4) Resize the new partition you have created (at the bottom of the diagram) to leave the amount of space you want for your Win install.
5) Set it to be FAT32, and call it something like "Windows" so you can identify it later.
6) Reboot with your Windows DVD in your tray.
7) When you get to the point where you select a partition, be careful to select the one you created earlier. As it is is FAT32 you can't install NTFS versions of Windows to it, so at this point reformat it to NTFS.
8) Install Windows on it.
9) You will notice after rebooting that you no longer boot to Snow Leopard, don't worry. Boot into Windows, install SP1, all your drivers etc, set up your screen res, reboot as many times as you like, get it set up the way you like it.
10) Boot from your iBoot CD again, it will take you into your old Snow Leopard installation.
11) Run MultiBeast and select NO OPTIONS except to install your choice of Chameleon bootloader - whichever one you had working previously.
12) From now on when you boot, you will go to the Chameleon menu. Press a key during the progress bar countdown to reveal the multiboot menu, and select either of your two OSes.