Try to do a search...is what everyone else is thinking whenever they see a post like yours. And while it's not bad advice, I still don't agree that new users ought not be helped because they didn't read the precious rules. (Yes, I know, it says it in the <blah-blah-blah-dios>...but did they really read all of the TOS when they joined? (...I didn't think so)
@AE86: Sorry for the little mini-rant. I didn't mean you you...you know? lol...At any rate, welcome. 3.1.3 cannot be jailbroken as of yet, and no, there is also no easy way to downgrade to 3.1.2. In fact, and without going into any more details, it is not possible at all if you've never had your device jailbroken. As for your apps, in theory, they would not be affected, and even if you were to find that all of your apps were gone (for any reason), you can re-download them again for free as many times as you want, so long as you log in to the account that you used to purchase them to begin with. This won't help you with your jailbreak, I know. I just wanted to answer all of your questions and I also see that you're brand new to these forums.
Hang in there. It will be possible soon.