Chronic Dev Team has released the initial version of greenpois0n for iPhone 4, iPhone 3GS, iPod Touch 3rd and 4th generations, and of course iPad!. They also mention of an upcoming version, supporting Apple TV and iPod Touch 2nd gen.
On iPhone 4, greenpois0n works with firmware 4.1. Even with lower firmware, they say you can use TinyUmbrella to preserve the baseband.
Chronic Dev team is using the controversial bootrom USB exploit by Geohot rather than the SHAtter Exploit.
You can download the greenpois0n from, where else, http://www.greenpois0n.com/
and follow the info at Chronic Dev Team Blog.