For those with the newest 3GS model, where ur seriel number, 4th and 5th digit are 41 and above, means u have the new bootrom thingy, which means is the so-called tethered jailbreak.
so when u reboot or shutdown or installed app that requires rebooting, u will go into recovery mode and can't do anything but dun panic. Just execute the blackra1n again and make it rain, it will reboot it back as before. So while u are outside, dun simply install app that reboots or turn off ur iPhone.
Another problem is that, sometimes can become No Service and cannot do anything. To steps to solve it is to take out ur sim card and insert back while ur iPhone is still turned on. This is for the case if u are outside and don't have computer. So must always bring a pin around. If you have computer with you, u can shutdown it down, then blackra1n boot it up and can get ur network already..
Hope this helps, anyone got any fix or solution, please share ya. I read in some post saying turn off and on the 3G, Edge or Airplane mode, but all no use