I'm sure I could simplify this procedure, but this is what I did, and it worked, so I'm just going to keep it the same. No purchase of any fancy software required...
1. Connect your Seagate drive that only works in Windows
2. Open Disk Utility and "unmount" the partition (do not eject). Highlight the Seagate drive and click the "i" Info button at the top. Note where it says "Disk Identifier : disk#" (mine was disk2).
3. Open a Terminal Window (/Applications/Utilities/Terminal.app
4. Run the following command "dd if=/dev/disk# of=mbr count=100". "if" means "input filename"; "of" means "output filename". Fill in the '#' with your disk number from step 2. The "count=100" is just to be sure we grabbed enough. We don't actually need 100 blocks, but it doesn't matter.
5. Download HexFiend from one of these websites...
http://www.ridiculousfish.com/hexfiend/
http://www.macupdate.com/info.php/id/22589
6. Open the file "mbr" in HexFiend. Note that most everything is "00000000", except a handful of other "cells". These must be blocking MacOSX DiskUtility from being able to repartition. Replace anything in the beginning that's not "00000000" with "00000000". Click "File->Save As..." and call it 'newmbr'.
7. In the terminal window again, run "dd if=newmbr of=/dev/disk#" (remember to use your disk number from step 2).
8. Now you can open DiskUtility again and repartition the drive just as you should have been able to do if Seagate wasn't blatantly trying to milk Apple's customers by charging more money for the same thing.
9. Your done! Enjoy your cheaper "Windows-only" Seagate hard drive.