Let's clarify something, just in case: We don't call CHDK firmware. We call it software. What we call firmware is Canon's original firmware that lies inside your camera. For some cameras, Canon has published firmware updates. For all CHDK compatible cameras, we have read Canon's firmware from the camera's memory to learn how to run software (CHDK) on top of it.
When you install CHDK, you do not replace or modify firmware. You just put a program (CHDK) on the SD card and run it (either using autoboot or by fooling the camera to think you have a firmware update on the card when in reality you only have CHDK there).
So, are you trying to install an a620 version of CHDK on your a610? It will not work, and you shouldn't even try it.
Or, have you really done what you say you have i.e. found an official or unofficial Canon firmware update package for a620 and somehow managed to install it to your a610 (which sounds just plain insane)?
Now,
1) take your SD card out of the camera.
2) take the batteries out of the camera
3) take the backup "coin" battery out of the camera
4) wait a few minutes
5) put all those batteries back and then power on your camera WITHOUT THE SD CARD. If the camera is still broken, I believe you really bricked it (unless the batteries are dead).
If the camera works, just format your SD card and follow CHDK install instructions more closely.
