i have an iPhone 3g and don't want to risk updating firmware on account of losing most of my info... is there a solution or software that could help me use ios 4.0 apps on my 1os 3.1.2? I would be surprised if there wasn't!
You won't lose any data by updating. Just be sure to sync and backup in iTunes before updating.
If you still don't want to update, you won't be able to use these apps if you're iPhone is stock. If it's jailbroken, you can install Fake 3.1.3 in Cydia to allow installation of apps that work on 3.1.3, but not 3.1.2 (such as Netflix). I'm not sure if there is an app like this for 4.x.
You can sync a jailbroken iPhone with iTunes with no problems. Not backing up is risky because you'll lose a lot if the phone crashes or is lost. The iTunes backup could be used on a replacement phone or after restoring. A common misconception is that it will automatically update the firmware. It asks you if you want to update and you have the option of declining. I recommend updating as long as your phone isn't unlocked. If it is unlocked, you'll need more specific instructions to retain unlockability when updating.
You'll have to rejailbreak after you update, but you can backup your jailbroken apps with PkgBackup. You don't need to restore. Just hit the update button after syncing and backing up. All your apps, settings, data, bookmarks, contacts and media will still be there. It will update the 3G to 4.2.1 and then you can rejailbreak with limera1n or greenpois0n.