Yes, Apple is releasing it today. No you should not update to it.

For those that did not read that, DO NOT UPDATE TO 4.2.  Ok now that you’ve read it, wait for the jailbreak to be officially tested and released. Further, remember that jailbreak on 4.2 is likely tethered on newer devices, which means you have to rerun it every time you reboot your device.  This was covered here already. Dev team also covered this in more detail here.