According to the Dev-Team Blog, it looks like Apple might be making the practice of downgrading iOS using SHSH blobs, much more difficult. SHSH blobs are something stored by Jailbreak store Cydia and utilities like TinyUmbrella and used to get around Apple's typical requirement that every firmware installation be signed by Apple's. The Dev-Team says this has to do with the process behind over-the-air (OTA) updates, but either way it's not great news for frequent SHSH blob users.
Starting with the iOS5 beta, the role of the “APTicket” is changing — it’s being used much like the “BBTicket” has always been used. The LLB and iBoot stages of the boot sequence are being refined to depend on the authenticity of the APTicket, which is uniquely generated at each and every restore (in other words, it doesn’t depend merely on your ECID and firmware version…it changes every time you restore, based partly on a random number). This APTicket authentication will happen at every boot, not just at restore time. Because only Apple has the crypto keys to properly sign the per-restore APTicket, replayed APTickets are useless.
This will only be in place for iOS 5 onward, and only when Apple makes use of it. It also can't stop limera1n, which happens before these checks in the boot sequence. What, if any, workaround may be possible the Dev Team doesn't say... and won't until iOS 5 is out of beta.
Whether or not Apple is actually targeting Jailbreak with this is difficult for me to say. I've often conjectured that Apple enjoys the "incubator" that is Jailbreak -- a way for para-iOS features to be tested on a large user base without having to sanction it, and a way to serve the hardcore market without having to support it. They've even ignored exploits in past iOS point updates. We'll have to wait and see if a) Apple implements it in furtherance of OTA updates, b) if the Dev-Team finds a way around it, and c) how Apple responds to any circumvention.