I've just reinstalled the latest jailbreak from mobileread. I then deleted my keystore from the device and reinstalled it from the zipfile downloaded from my adqmisc googlecode project. Result: works fine.
Obviously something is affecting some people though, but as I can't replicate it, I'm at a loss as to what it is. I can only suggest a few things to try out:
1) it could perhaps be some of the other files in the keystore directory. If someone wants to try sshing in and :
rm .* *
That should wipe everything. Then reinstall my devkeys (the above will zap them) and try again.
Update: I've created a standard kindle update that will do these steps, available here. If you're using the latest jailbreak, you'll need to add "update_zapdevkeys" to the end of "<Kindleusb>/linkjail/etc/whitelist,
I can't guarantee it'll help as I can't replicate the problem, but its the only thing I can think of. You'll still need to reinstall my devkeys afterwards. Please let me know either way!
2) Another thing springs to mind: check the date on the kif.azw2 file on the kindle's USB exported filing system; perhaps the kindle is checking this against the key somehow?
Update: I don't think its (2) - just tried setting the date on the file to 1-jan-1970 and 1-jan-3000, but it still loads fine here.