The limera1n software is purportedly an iPhone 4.0 jailbreak tool by geohot. He registered the name limera1n.com shortly after he announced his untethered jailbreak. You may be able to jailbreak 4.0 for iPhone 4G, iPhone, iPad, or iPod touch with limera1n.
The limera1n download is not out yet, however you can click here to download iPhone toolbar now and be notified as soon as it's released. The source code of the limera1n website indicates that Geohot recently updated his website and added a Google Analytics code now that his software has already gotten a lot of buzz. You can expect limera1n release date to be in June because Apple may patch the exploit if Geohot releases it earlier.