Two issues -
(1) Am I the only person that thinks putting your backups on the drive you’ve also got your data on is a mistake?
(2) You can backup your Mac to MBL using Time Machine but I don’t consider this option to be very reliable. Can’t comment on the Windows side of things.
When I was using MBL for my TM backups the MBL was all over the place, it would rarely sleep, the green light was flashing constantly and the TM backups were to say the least very erratic. Sometimes they would race away then suddenly grind to a halt, sometimes they would start off slow then speed up, then other times they would crawl for ages then hang.
Now I have started using a small portable USB HD for TM backups it’s a whole different world. TM backups are quick smooth and flawless and MBL is working like I envisaged it would. It sleeps without incident and doesn’t wake up unless accessed by me or another user.
It could be that with further firmware updates TM may work better with MBL, though unless you have the Duo there’s still the question of should you be using the drive for backups and data storage.
BTW OS is Lion fully up to date and MBL has latest firmware.