Custom killstreak details for Modern Warfare 2 emerged thanks to a preview in the latest issue of PSM3 Magazine.
Looks like MW2 allows for mixing and matching rewards to fit your skills, which means that newer players can look forward to getting rewards for smaller strings of kills. Those who are considered "veterans" can receive higher-end rewards.
"You can have a three killstreak or a fifteen killstreak if you want, or something like the AC-130 [firing down in black and white from an airborne gunship] is one of the highest killstreaks you can select," Infinity Ward's Robert Bowling told the mag. "You take control of that and can fire down for thirty seconds, blow guys up."
There's a video through the link showing this off, which you have all see before, but there it is anyway so as to put this bit into perspective.