DFU must work if the hardware is ok. Google for iRecovery the software talks to iBoot

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iPhone 3GS factory unlocked (3.1.2; Blackra1n, Cydia, OpenSSH, custom 3.0 ipcc file (no sig), tethering hack by Dev team, 32 GB)
iPhone 3G (3.0.1; Redsn0w 0.8, Cydia, OpenSSH) 16 GB

take out the sim and put it back. and keep switching on and off

you are probably not in the proper DFU mode, it happens, you have the black screen, but you're not in DFU,
have you tried restoring to the latest version again and running quickpwn or tried running quickpwn alone?

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I'm going to give it a try .. who knows!

I can't restore to any version. When I run quickpwn, it tells me that the firmware version which I'd used does not match the version of my device which it sees not as "3.0" but as "_3.0" ahhhh!! Could this modified version of QuickPwn have done this? Is there a way I can have quickpwn ignore the version number that my phone seems to be reporting and pwn it again anyway??

The main thing here is to get the phone in DFU and get itunes to recognize the phone.
then restore it with apple original firmware 2.2 to be safe.
If you cannot do this, something is really wrong.

Google for irecovery like Volkspost suggested.

If you can get the original firmware back onto the phone, then you can jailbreak it later

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i had a problem with quickpwn saying that and i went onto there site and downloaded the full version and went through the process, this worked ok

To get iPhone in DFU, hold down the Power+Home buttons for not one, but a second reboot, and continue to hold and it will tell you to connect your device to iTunes via USB.

However, does anybody know any kind of jailbreak for the official 3.0 release? Stupid me actually upgrading from my 2.2.1 that worked perfectly fine =).

Caution: Those who've noticed the official MMS2G installation app through Cydia, do not download and install it if you want to keep your current FW. This will cause your device to remain in "Recovery Mode" with the Apple logo until you do go into DFU. Have fun Recovering your device through iTunes like I did *sigh*