There's an iOS4 jailbreak available and it's by far the easiest I've ever done.  Just point Mobile Safari on your iOS4 device to http://www.jailbreakme.com and follow the instructions.  My iPhone 4 took just a little over a minute to jailbreak and reboot.  Viola!  Cydia installed and all the non-Apple-approved goodies available.  Just one word of caution:  DO THIS AT YOUR OWN RISK!  Apple has already stated that despite the recent DMCA ruling, jailbreaking voids your warranty.