Just a quick post explaining a problem I found when upgrading my jailbroken (using spirit) 3.1.3 iPhone 3GS to iOS4.
I did the upgrade last night, and noticed this morning that my data network didn’t work at all. WiFi was working fine, but network data wasn’t at all.
After a bit of googling, I came across this post which states that it is due to the jailbreak 
So, to anyone running 3.1.3 jailbroken, here’s how to upgrade
First, do a restore rather than an upgrade (click restore in itunes and it will basically do a clean install of iOS4 then resync everything) – you won’t lose any data. This should work fine and you won’t have any data issues.
If you did a normal upgrade rather than a restore, you must have a backup pre iOS4 to be able to get it working again. In this instance, simply restore the device and then restore the backup pre iOS4. For more information on how to save multiple backups at any time, check out my earlier blog post.
If you did an ‘upgrade’ and don’t have a backup pre iOS4 any more, it looks like you’re going to have to do a clean restore and not use a backup. You can get some of your data (SMS etc..) back by using the advice in the link above.
Good Luck!
