It’s incredibly simple to do the jailbreak. Just goto www.jailbreakme.com on your iOS4 device and “slide to jailbreak”. Cydia will be on the homescreen after a couple minutes of downloading and installing.
Not much else to say about this other than it worked perfectly for me on my iPhone 4.
As always, backup before doing anything like this to ANY device you own. Oh.. and this can brick your phone – rare of course – but it can happen.
If you decide that a jb’d phone just ain’t your thing.. just plug it into itunes and restore.