If the phone's bootloader is still intact, that is you see at least the white Galaxy S/Samsung/AT&T logo or the "PC ! Phone" logo on the screen, you should be able to force it into download mode by connecting a homemade jig that simulates the Samsung "Anyway" field test jig. You can buy these jigs online, or make them using a micro USB plug and a resistor.
If your phone screen is always black, that is it does not display the white Galaxy S/Samsung/AT&T logo when you switch it on, then sorry, this jig will not help you.
If this helps you recover your bricked phone, please donate to the CyanogenMod developers: go to this thread and click the link under "SUPPORT US". Thanks.
If you don't want to buy a pre-made jig, you can build one yourself from a few parts. You will need:
1. a micro USB plug that you can solder onto. If you are not confident with soldering or you just want a tidy solution, buy this micro USB breakout board from Sparkfun.
Otherwise, get a cheap micro USB cable and cut the sleeving off the plug with a sharp knife. You cannot just wire onto the cable because this jig uses a normally-unused pin (pin 4) of the micro USB plug which will not be connected to the cable.
2. a 301k resistor. You can buy one from Digikey, or make one up by joining several resistors together in series, for example 3x100k and 1x1k. The resistor value does need to be fairly accurate (I would guess within 1% of 301k) to trigger download mode. 300K seems to work for everyone so far.
1. Solder the resistor across pins 4 and 5 of the micro USB plug. Do not do this with the plug plugged in to the phone as the heat could damage the phone's USB socket. The pins are shown in this diagram ("Micro-B"). On the Sparkfun micro USB breakout board they are labelled "ID" (pin 4) and "GND" (pin 5). When the plug is plugged in to the phone they will be the pins closest to the headphone socket.
2. Make sure the phone is fully switched off by pulling the battery, waiting 5 minutes, then replacing the battery. Do not switch the phone on.
3. Connect the rigged USB plug to the phone. Try not to touch the pins on the back of the plug or the resistor(s) as you are plugging it in, as your skin resistance could affect the resistor value seen by the phone. You do not need to press the power button. After a few seconds the phone should go into download mode.
4. Disconnect the rigged USB plug and connect a regular USB cable from your PC to the phone, then you should be able to flash using Odin.
For more information on how this works see this thread and this thread.