Death becomes Lost

Lot of speculation brewing in the world of tv discussion/spoilers that a major character will die before the end of the season.

Oh no! I don't know if there's anyone I can part with!

I have some spoilers, hints at who it might be, and not be. If you want to know....highlight to reveal below:

1. A current series regular is getting killed before the end of season five.
2. The character who will die is someone who is much-loved by many of you fans.
3. This is a real leaving-the-show death, à la Shannon or Charlie, not a quick-fix time-travel death like Jeremy Bentham/John Locke.
4. There was some speculation that it was Kate, but it is apparently not.

Thoughts?