welcome to the forum, if you look around there are tutorials on how to downgrade. but imo its better to just wait for a 4.0.2 jailbreak, its not the end of the world with out it unless you rely on unlock
If you want to learn anything about downgrading or jailbreaks, go to Dev-Team Blog (the guys that make the jailbreaks/unlocks).
I don't beileve that they are going to release a jailbreak for 4.0.2 because that firmware was released only to fix the exploit from jailbreakme.com v2.0
The next jailbreak won't be out until 4.1 (I think).
Correct me if I'm wrong.
but now its legal you might see more legit companies charging for it. but i wouldent risk it. not that im going to ever jailbreak or unlock but for those who want it. just read reviews and make sure you do your h/w on the site make sure its legit.