redsn0w 0.9.8b2 http://is.gd/6eek4Y should fix 50b3 App Store issue (if you're already JB, just use this to boot tethered)
Jailbreakme.com also received an update from Comex that's being pushed out via Cydia. Saurik explains:
So, @comex made some fixes to the kernel patches for JailbreakMe, and we are pushing them as an "Essential Upgrade"... I hope this works! :(
To be clear: do /not/ go back to jailbreakme.com and "redo" the jailbreak; instead, just open Cydia, and accept the new "Essential Upgrade".
As always, jump into our Jailbreak Forum for more info and help.