Q: how do I know I have a bad HD?

It may not be the hard disk. Try an SMC reset:

Resetting the SMC on Mac portables with a battery you can remove

1. Shut down the computer.
2. Disconnect the MagSafe power adapter from the computer, if it's connected.
3. Remove the battery (to remove the battery - click here: MacBook or MacBook Pro).
4. Press and hold the power button for 5 seconds.
5. Release the power button.
6. Reconnect the battery and MagSafe power adapter.
7. Press the power button to turn on the computer.

Resetting the SMC on portables with a battery you should not remove on your own

1. Shut down the computer.
2. Plug in the MagSafe power adapter to a power source, connecting it to the Mac if its not already connected.
3. On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time.
4. Release all the keys and the power button at the same time.
5. Press the power button to turn on the computer.

OK thanks. Please post your results, if no success then we'll consider HD problems and solutions.

Are there any signs of life at all? Any noises?

If the answer is no, Apple offers little guidance beyond the obvious (plug it in etc), and what you just did. Read this to be sure I didn't miss anything:

Troubleshooting: My computer won't turn on

The $65 may be worth it. Apple applies it to the cost of repair should you decide to do that. A problem like this may range from the very simple (a bad battery connection) to uneconomical to repair (a defective logic board).

With no signs of life whatsoever you cannot even boot from the system installation DVD or run Apple Hardware Test.

This is one for the Genius Bar.

That is really for you to decide. Read this iFixit guide to see if you're up to it:

http://www.ifixit.com/Device/MacBook_Pro

They sell logic boards as well, but in very limited quantities. The part itself would be about half of what the Genius Bar quoted for a complete repair. You would have to be absolutely certain the part is an exact match.