If you are running WordPress as your blogging platform and if you have been trusting enough to leave User registration enabled for guests, DISABLE IT IMMEDIATELY (in wp-admin >> options: make sure “Anyone can register” is not checked).
Additionally, delete or disable ANY guest account already created by people you are not sure about.
Leaving it open and letting people sign-up for guest accounts on your WordPress blog could lead to incredibly nasty stuff happening if anybody so desired. And trust me I am not exaggerating this. So don’t wait a second to disable this option and please relay the message.
WordPress dev team has been notified a while back and I dare hope they will soon start acting on it, if only by relaying a similar announcement through the official channel (as well as, of course, releasing a proper patch).
Sorry for the shrill hysterical tone, but this is a big deal. However, disable that one option and you are fine, no need to panic further
[cheers go to Geoff Eby for discovering and bringing this insane security exploit to my attention]
Update: a small follow-up addressing comments and concerns I have received ever since this last warning, is posted here. Feel free to ignore completely unless you really care about inner WordPress politics (yawn).
Update 2: WordPress upgrade 2.0.4 should now patch this bug. If your version of WordPress is equal to or higher than 2.0.4, feel free to ignore the warning above. If not, then you should/MUST upgrade (more details in the comments).