Well, although it sounds a good idea, it's not.

Like the other guys said it's not like the terrorists will have www.bombtheusa.org/plans or whatever so finding a site of use will be hard. But, and this is my reason for not going after terrorists\nazis\kiddie sites and simmilar.
Imagine you find a "target" and start hacking in and messing with it, you find some good info so download it and trash the server then send the info to the feds. Woohoo good job...untill they tell you that they've had it under survalence for months, that every wire in or out is bugged, that they thought they had saddams IP from his secret bunker.

Ok way over the top but you get the picture, IMHO it's not a good idea.