This just happened to me. I fixed mine by using odin to go back to DK28, then installed CW3, then installed midnight off of SD card from CW3.
Start with the phone off, open the slider and hold the 1 key down and then hold the power button. Does the yellow triangle eventually come up? Be sure and read the Odin howto carefully. This is a pretty hard phone to brick, you may have just missed a step somewhere.
[How to] Factory Restore (or Fix a Bricked Phone) - Android Forums
In my case I think the problem was caused by trying to go from eclair to midnight rom. I'm guessing that midnight doesn't install any radios or modems and the eclair versions don't work with it.
By the way, so far midnight rom looks pretty nice. this is my first custom rom on the new phone, I may have picked the right one. I figured if Darren was using it it must be pretty good.