The iTunes 7.4 upgrade came as bad news to some iPhone hackers. It "broke" many existing hacks, including jailbreak applications and ringtone installers. But the news is far from bad as developers are quickly working to adapt to the change.
Nullriver's AppTapp Installer.app has already been updated and is working with 7.4, according to its developer "lg". Programmers for Breezy and iBrickr are also working on their updates at this time.
7.4 apparently changed several functions that were used to access the iPhone file system but it doesn't appear to be a backlash from Apple or any intentional thwarting of hacker activity.
Update: To clarify, this problem seems to be limited to new jailbreaks and restored systems. I'll keep updating as I find out more. Breezy does not jailbreak but it's being looked over thoroughly by the devteam. iBrickr developer Nate True reports that all features except ringtones-without-jailbreak are working. He expects that ringtones-without-jailbreak are broken for all apps including iToner and iPhoneRingtoneMaker, regardless of software and that they will not survive a sync. The reason is that these apps use the iTunes folder to store ringtones and a property list and are getting overruled by the new iTunes ringtone-compatible update. Ringtones installed into ~/Library/Ringtones instead are not affected.
I'm also hearing that the alpha version of the Windows AppTapp/Installer.app is being tested right now.