I was going through some of the source code for MW2 that got leaked on the internet a while ago, and it looks like there was a tank killstreak that was created but never used.
It doesn't look like it was finished tho, it refers to a header file that doesn't exist (although it has been commented out).
The kill streak gives you a tank you control, which has a turret, machine gun turret and 3000 health. In comparison a person only get 100 health.
As the tank gets damaged the slower it moves (this is also commented out)
I think its good they decided to not use a tank as a killstreak.