Apple has roll out iOS 4.0.2. No new features. Just a patch to cover a vulnerability on opening PDF files. As you may already know, the Jailbreak Dev-team uses this to tunnel into iPhone OS and jailbreak it.
If you upgrade to iOS 4.0.2 now, you will NOT be able to jailbreak your iPhone anymore (at least until the Dev-team found another solution).
So, if jailbreak is a new term for you, you probably can upgrade this patch. If jailbreak is the ONLY thing that you think is great, don’t upgrade now. Wait for new development from the Jailbreaking team.