(This is mostly a record of what I had to do to get my computer sorted out. I'm sure there are better ways, this is just what I had to work with.)
Step 0. Back up your data. Noob.
Step 1. Use Windows to create a rescue disk (go to Go to Start Menu > Control Panel > System & Security > Backup and Restore > Create a System Repair Disc).
Step 2. Shrink your Mac OS X partition using disk utility (Macintosh HD > Applications > Utilities).
Step 3. Download and burn the GParted live CD from http://gparted.sourceforge.net/livecd.p
Step 4. Download and burn the rEFIt boot CD from http://refit.sourceforge.net/
Step 5. Boot from the GParted CD and expand your boot camp partition.
Step 6. Boot from the rEFIt boot CD and hit 'p' to run the partition tool. It should say that the GPT and MBR are out of sync and ask if you want to synchronize them. Hit 'yes'.
Step 7. Reboot and hold down the option key. Your windows partition should be visible now. Try booting from it. If you are successful, you're done. If it says that the boot information is invalid, continue to step 8.
Step 8. Boot from the System Repair Disc you made in step 1. (You may need to boot into Mac OS X and run the bootcamp assistant. Tell it to boot from the windows install disc.)
Step 9. Click on 'repair this computer.' It should say that the boot information is invalid and prompt you to update it. Do so.
Step 10. Reboot into Windows. If it still fails, rerun the repair. You may need to run it up to 3 times, as there's 3 potential problems with booting, and the repair disc only fixes one boot issue at a time.
Alternatively, plunk down $20 for camptune and save yourself an afternoon.