iPod Touch 2G – iPod2,1_4.2.1_8C148_Restore.ipsw
iPod Touch 3G – iPod3,1_4.2.1_8C148_Restore.ipsw
iPod Touch 4G – iPod4,1_4.2.1_8C148_Restore.ipsw
iPad 3G/WiFi – iPad1,1_4.2.1_8C148_Restore.ipsw
iPhone 3G – iPhone1,2_4.2.1_8C148_Restore.ipsw
iPhone 3GS – iPhone2,1_4.2.1_8C148a_Restore.ipsw
iPhone 4 – iPhone3,1_4.2.1_8C148_Restore.ipsw

Again, there isn’t an untethered jailbreak yet, so avoid this if you’re a jailbreaker and like to actually use your phone without carrying your laptop around with it. As always, avoid this for now if you rely on the carrier unlock as well. Upgrading the iOS will also irreversibly upgrade the baseband (modem firmware), which could permanently remove your ability to unlock the phone.