I have FINALLY succeeded at jail breaking my iphone 3g 4.2.1 (i think!)... I used redsnow (redsn0w_mac_0.9.6rc16) with the regular and custom ispw's. My baseband had to be upgraded to 6.15 first as it was 5.15 previously, and it took me ages to finally figure out the jailbreak, it kept getting stuck in emergency call mode. ANyways, Now I can finally access the iphone, but when I try and run cydia it just opens momentarily in a blank screen and then returns to the main app screen. Any idea how to fix this? I am completely new to all this jailbreaking/unlocking language by the way so basic instructions would be much appreciated!!!