Yeah, I read an editorial on Mugglenet about what you're saying. That Dumbledore was just trying to protect Draco and knew that, because of the Unbreakable Vow, that Snape would die if he neither he or Draco killed Voldemort. So Snape and Dumbledore decided that it was for the best that Snape kill Dumbledore because Snape was too valuable for the Order to lose.

But then he refuted his own point by saying that, if it had been planned, that there should have been some indication given to somebody in the Order, because Snape can no longer be an effective spy (if he ever was really spying for the Order) if no one in the Order trusts him, which is how Book 6 ends.