Previously, it was possible for users to save their SHSH blobs to downgrade their version of iOS to a previous one. These worked by basically tricking iTunes into thinking that the version you were downgrading to was the current build.
"Starting with the iOS5 beta, the role of the ‚Äö√Ñ√∫APTicket‚Äö√Ñ√π is changing ‚Äö√Ñ√Æ it‚Äö√Ñ√¥s being used much like the ‚Äö√Ñ√∫BBTicket‚Äö√Ñ√π 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‚Äö√Ñ√¥t depend merely on your ECID and firmware version‚Äö√Ñ¬∂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."
However, they explain that tethered jailbreaks "will still always be possible for devices where limera1n applies." They also say that "restoring to pre-5.0 firmwares with saved blobs will still be possible" though they note that you will have to used older versions of iTunes. Here's how to do that.
Jailbreakers will always find a way. Apple will counter by plugging a hole and the cycle won't end.