greenpois0n is very iffy (if you know what I mean). But that's Chronic-Dev Team for ya! It's best to just use PwnageTool to create your own pre-jailbroken IPSW that you can restore to with iTunes. If you aren't on Mac OS X, then use sn0wbreeze.
Are you sure the phone is in DFU mode and not Recovery mode? DFU should give you the black screen but you should still be able to tell it's on. Also make sure when you connect your phone, iTunes doesn't come up. It shouldn't be active at all.
I literally just finished j/b my 3G before I put it up for sale... I tried using GP but had no luck... there's some sort of issue where gp won't recognize the phone is in dfu mode.. I went to pwnage and it worked perfectly.. I'd also suggest giving pwnage a try.
Hmmm... I guess it depends on a lot of factors... computer OS version as well as updates installed, etc. ... who knows. I just know it would always say "Try again" for me until I realized I wasn't really in DFU mode. Once I got it there, it went as smooth as butter.