I recently just upgrade to 4.2.7 and now my phone is tethered jb using redsn0w 0.9.6rc14.. it works perfectly fine for me but im still waiting for greenp0ison to release jb for 4.2.7..
Steps to jb using redsn0w:
First: download redsn0w 0.9.6rc14
2nd: download IPSW 4.2.6 (have to use IPSW 4.2.6 on 4.2.7 to make it work)
3rd: open redsn0w and browse for IPSW 4.2.6
4th: follow all the instructions, then just check "install cydia" .. let it finish and reboot
5th: turn off the phone again and plug it on pc
6th: run redsn0w again and browse for IPSW 4.2.6
7th: follow all the instructions again but this time just check "boot tether".. let it finish and your done.. phone is jb
Remember this is tethered jb, everytime your phone got turn off just follow the instructions from step 6 to 7 only to get it working again..
hope this helps you guys cuz it surely did on me