While it appears to have only been tested on an iPhone 3G and an iPod touch 2G (non-MC model) as of yet, it looks like Redsn0w can jailbreak the latest firmware released by Apple today, iOS 4.0.2. If you are interested in doing this, a step by step guide is posted below.
- Download iOS 4.0.2 via iTunes 9.2.1.
- Update your iPhone to the latest firmware you downloaded in step 1.
- Start Redsn0w 0.95b5-5 and select iOS 4 (Final Version), not iOS 4.0.2.

- Choose “Install Cydia”, followed by “Next”

- Now, Turn your device OFF and connect it to your PC before clicking next, as directed on the screen. (image below)
- Put your iPhone into recovery mode by following the onscreen instructions

- Redsn0w will now complete the process and leave you with a jailbroken iPhone or iPod touch running the latest iOS 4.0.2 firmware release
Download links: