I would like to know as well.
I recently downgraded from 4.0 to 3.1.2 after being unsatisfied with the WiFi dropping out every 10 seconds and lack of anything special on iOS4, I've successfully downgraded to 3.1.2 but can't get it to update to 3.1.3; only 4.0 (now 4.0.1) appear as updates.