There are ways to do it on the web, but it's illegal (obtaining the .ipsw without being a developer [which requires paying 100 bucks] is considered piracy). You're better of waiting for the final release as the betas are intended for devs and dev devices only to test apps, no matter how stable they may be. Plus, Apple does not service devices messed up from a beta. You're better of staying on 5.1.1 which will net you the best performance and battery life till 6.0 is released.
Again, there are ways to do it, but it's not suggested to be on the safe side (if you rely heavily on your device, care about battery life, etc).
The only reason i ask is....i used to have a dev account but dont anymore. I enjoyed having the ios5 betas last year. i didnt want to re-register this late into the beta cycles.