What iOS 5 brings to the table is the ability for Apple to patch your device with small updates, frequently, to address any issues. Unlike today, when to plug a security hole they need to coordinate an entire new release, iOS 5 will allow nearly instant, wireless updates for any update Apple wishes to push to your device.
Sure, you can go offline, but if you want to play in the iCloud, you will be open to instant updates of the OS that have the potential of breaking any jail broken device at any time.
This is likely to cause the casual jail breaker to quit the practice altogether.
It will be interesting to see if Apple cares enough about the jail break practice to address it in this way. I can tell you they are being hounded by the developer community to address the problem.
Whether they do or not has yet to be determined.