I know this is an old thread, BUT I had the same exact problem (along with a lot of others on the net) and figured out what it was (without spending $ on an apple tech)! When I put in my new HD, going from a 160GB to a 750GB, put the case back on, no power nothing. I took the case off and plugged the power chord in with no battery, and it started up. The problem was the thin little HDD flex connector that powers the HD. When I peeled back the connector to get it off of the old hard drive, I accidentally ripped it. It was at a "Y" in the connector near the top where the HD gets plugged in, so it was hard to detect and barely noticeably. When I went to put the case back on, the power died again because the top case compressed the stress point that ripped in that connector causing it to short and kill the power. I used some scotch tape to tape it in place at the point that was torn. The part number for mine is 821-0513A if you want to look it up to see what I'm talking about. If anyone else has this problem, I suggest thoroughly checking every point of that connector. It's about a $30 part brand new.
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Maybe this link with a picture of the part I am talking about will work. Its a very fragile connector:
I know this is an old thread, BUT I had the same exact problem (along with a lot of others on the net) and figured out what it was (without spending $ on an apple tech)! When I put in my new HD, going from a 160GB to a 750GB, put the case back on, no power nothing. I took the case off and plugged the power chord in with no battery, and it started up. The problem was the thin little HDD flex connector that powers the HD. When I peeled back the connector to get it off of the old hard drive, I accidentally ripped it. It was at a "Y" in the connector near the top where the HD gets plugged in, so it was hard to detect and barely noticeably. When I went to put the case back on, the power died again because the top case compressed the stress point that ripped in that connector causing it to short and kill the power. I used some scotch tape to tape it in place at the point that was torn. The part number for mine is 821-0513A if you want to look it up to see what I'm talking about. If anyone else has this problem, I suggest thoroughly checking every point of that connector. It's about a $30 part brand new.