I guess I don't know how Installous works as I've never installed cracked apps. It seems like you're saying that it takes the real IPA's or something and installs them rather than having someone already crack the app and turn it into a .deb file? I was more referring to the later because it'd be very easy to bundle up a trojan with the .deb of any app not downloaded from it's original repo. Someone would only have to download a legitimate app, and compile a .deb with a trojan included. This is why you're safe on cydia sticking to the main repos.
Ah, now I see why we were talking past each other.
Cracked apps are usually distributed as .ipa's not .deb's. Cracked apps are cracked by an individual, and then distributed in cracked, unencrypted form. They can be installed through installous, or even iTunes, which does not grant them sufficient privileges to do anything terribly malicious.
I wouldn't really call sticking to the main cydia repositories as "safe". If I really was some sort of malicious hacker, and I wanted to attack jailbroken iPhones, what I'd do is write some sort of useful free jailbreak app (such as Backgrounder or an SBSSettings extension for example) and give it away. I'd load this up with whatever malware I could conceive of. I believe this would be the simplest, most effective approach. Definately a lot easier than cracking an app, and then finding some sort of exploit to get away from the iPhone's app jail.