1 Shot 1 Kill

Over on the IW forums, IW_Scriptacus was kind enough to share a little bit of groundbreaking info. Killstreaks have become stackable. Ie. If you have a 3,4, and 5 killstreak, you can not use your previous killstreaks and get past 5, but you will still be able to utilise them after you use the streak you just got.

3 - UAV
4 - Care Package
5 - Predator

Say you got 5 kills in 1 spray. Would you only be able to use the predator? No. Once you use the Predator, you can then use the Care Package, and once you use the Care Package, you can use the UAV. Kills gotten by these streaks will NOT count towards your current streak however.

The killstreaks stack and are saved. The stacking is what is called a LIFO (or FILO) queue. Last In, First Out. http://en.wikipedia.org/wiki/LIFO_(computing)

So for example:

3: UAV
4: Care Package
5: Sentry Gun

You can earn all three without using any and the one shown on your DPAD would be the Sentry Gun. As soon as you used it you would see the Care Package on your DPAD. As soon as you used that it you'd see the UAV.

Yes, you would still have them. However, any kills they were to get wouldn't count towards your killstreak during your new life. (this prevents exploits such as using you AC130 to get another AC130)