For example the case is, my iPhone is 4.2.1 Jailbroken. Since 4.3.1 Jailbreak is out, I want to update to 4.3.1 and jailbreak it too, but I don't want to lose all my apps.

This is how you should do: (still work in progress)

1) 4.2.1 (jailbroken)
2) Backup all of those data(apps,songs etc)
3) Upgrade to 4.3.1
4) JAILBREAK FIRST.DONT RESTORE BACKUP YET
5) Sync/restore your iPhone

Urgent action required for anyone that likes jailbreaking 4.3.1 or 4.3.2 and wants to do so in the near future.

For those of you unaware of the Apple firmware update policy, you can only restore firmware version that Apple approves. When you restore in iTunes, it goes to Apple’s web site and verifies if you are eligible for this specific version. This was solved by Cydia storing your SHSH (the is it ok to restore this version request response from Apple). This has been ok for the last year.

However, a couple weeks ago, Apple banned the cydia servers from getting shsh’s. Saurik has to reengineer this solution and is doing so. But in the meantime, Apple is still signing 4.3.1. I suggest you grab the updated tiny umbrella v4.30.05 (windows) (OSX) and save your SHSH locally for 4.3.1 on all your devices. For current 4.3.2 SHSH, use the latest tiny umbrella version (same instructions) (Windows) and (OSX).

To do this, go to advanced, uncheck “request shsh from cydia” and uncheck “set hosts to cydia on exit”. Then restart umbrella, go to show all shsh’s and save them all. In the logs you should see 4.3.1 come down. This will allow you to restore 4.3.1 or 4.3.2 through umbrella’s tss server if you need to.

Cydia’s SHSH system will hopefully be back up soon. When it is, I will post about it on this site. (Note: your older shsh’s are still safely stored in Cydia and are not lost).

Alternative to this is to install iSHSHit from Cydia, back up your SHSH and then email it to yourself. From there you can copy it to tinu umbrella’s SHSH folder.

Yes, if you ever in the future want to downgrade to a version that can be jailbroken, then save your SHSH. I also have 4.2.1 saved in Cydia, but not 4.3 eventhough I was jailbroken at 4.3.1/4.3.2

I've saved my 4.3.1 and 4.3.2 locally using TU and I recommend you do the same.

Say that Apple comes out with iOS 4.4 and you/your friend/your girlfriend accidently upgraded to 4.4 and there is no jailbreak out for 4.4, then you can use TU in conjunction with the SHSH that you have saved locally to downgrade to 4.3.2 and then jailbreak that with redsn0w. Without the 4.3.2 SHSH, you will not be able to downgrade to 4.3.2 once the 4.4 is out. See why saving SHSH is important?

There are many instances where you will need a downgrade such as when Apple comes out with a higher firmware version and your iPhone dies or something went wrong with the phone. The only way to bring the phone back is to restore it to the latest iOS firmware (which, let's say for arguments sake is not jailbreakable at the moment), what do you do? You will need to downgrade and you will need to have the SHSH ready.

It doesn't hurt to save the SHSH, I save them all the time (even for my Ipad 2 even though there is no jailbreak for the iPad 2 at the moment coz you never know what firmware version the dev-team will be able to jailbreak in the future)

Yes

Yes

Just make sure you go to Advance Tab and UNCHECK the box that clearly says: "Request SHSH From Cydia"

then click 'Apply Changes'

then select your device then click 'Save SHSH'

The next time you open TU, it will automatically find the saved SHSHs

Are you on 4.3.2 Redsnow?
http://twitter.com/#!/i0n1c/status/60378631385518080

Just unlucky bro happens to a couple of people. Maybe later on have fix for this

Have you tried to restore to a clean OS and try it? If you did then it's a hardware problem.

If it can then we wont need to ask you to save your SHSH So Save your Unique SHSH