Is the phone bricked?
Can you boot into Android?
I had a similar problem
http://xdaforums.com/showthread.php?t=748475
If bricked let it boot to the black screen, leave it until the phone turns off then try to enter DL mode with the key Combo
If not bricked and you can boot to Android, best way is to turn debugging on and use the adb to reboot into download mode
Mine is not bricked. Boots into Android fine. Just cannot access download mode, recovery mode, or hard reset, using the hardware buttons (which do work in android itself.)