Hacking a site and replacing it with your own content is a higher level attack than most. Usually hacks are about stealing information or taking a site down. As for the first one, the best that you'd be likely to get from the site is the BOE letters, which are always leaked anyways.
What I'd like to see (and I posted this in another thread about the site) is someone write a script to basically spam the "request a visit" form with random information that appears valid. Realistically, it wouldn't even need to be spammed all that much to make a significant impact upon the minds of the brothers in the congregations. If half of the visits they go on turn out to be people who claim that they didn't ask for it (or, as the case may be, commercial addresses or vacant houses, etc) then the R/F will start see that 'mother' isn't infallable. If you spam it enough, it would likely result in them throwing away real requests, which would put a damper on their recruitment efforts.
Another attack would be a distributed denial of service (DDoS) attack, in which you just get a bunch of computers to make page requests (or even just ping the site) overloading the servers. The trick here is you need a large number of computers to execute the attack, and this in itself requires either a large number of people willing to risk getting in trouble with the law, or someone who's gotten trojans on a lot of computers and can use them to carry out the attack.
