Joss and the writing team really thought the season out to a T. Then again they were planing on everything ever since season 3, so they had lots of time to prepare.
That's one of the reason why season 4 sucked, they were just thinking about 5!
I watched "the Gift" last night, and the episode was amazing. One thing that I noticed was that Buffy seemed happy to jump and die.
Somtimes I believe that the show should of been over after season 5 because of how poetic everything was. However, one of the main themes of the whole series was that Buffy would always be alone, she was the chosen one. In "Chosen" that was no longer the case, she didn't have to worry about that anymore.
Where did they say Buffy was last? They said it during Angel this year, but I can't seem to remember.