This may well be too late to be helpful, but it is quite possible to back up a Boot Camp volume via Time Machine. (And the previous answerer clearly didn't even understand the question, since he appears to be talking about writing backups to an NTFS drive.)
In fact, it's really easy. Rename the 'BOOTCAMP' volume to anything other than 'BOOTCAMP' and Time Machine will recognize it as a back-up-able drive. Then go to the Time Machine preference panel and click on 'Options...' You should now be able to select your NTFS volume and remove it from the exclusion list. Next time you run a backup, it will back up the NTFS drive.
HOWEVER: It may not be possible to restore a bootable NTFS drive from a Time Machine backup, due to permissions issues, metadata, etc. (Restoring would involve installing NTFS-3g or otherwise mounting an NTFS volume as read/write, and then restoring files to that, too, so remember there's an extra step.) Given that, though, it's a good way of saving your data files on Windows, if you have enough room on your backup drive.
If I have time, I may try a full restore from my backup to my boot camp partition. If I do, I'll record the results here.
EDIT: This may only work with 10.6, and/or may only work if you have MacFUSE and NTFS-3G installed.