Can I use Time Machine to back up data to a Windows shared drive?
In order to create a Time Machine backup to an unsupported drive (pretty much anything that's not formatted HFS+) you need to run the following Terminal command:

defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1


Your share will now show up as a valid Time Machine destination in the Time Machine Preference Pane.

Time Machine will then create a disk image on that drive that will act just like a HFS+ drive for Time Machines purposes (hard linking, etc.). One advantage of using a disk image over using the entire drive is that you can limit the size it will grow to of the disk image using Disk Utility.

Be sure to check Louis Gerbarg's answer as well for why it's unsupported and you have to jump through a hoop to enable it.