If its still under warranty, and the seal hasn't been cracked, contact Microsoft
Try what hijackthis1 suggested, if that doesn't fix it, try this.
Since you have the 360 apart, and the top cover off of the dvd
drive, here we go...
There's a circular cut-out that's glued to the top cover that you removed from the dvd drive, right below the sticker with all of the drive information. I was able to pop out the cut-out. After that you now have (3) pieces in your hand, the cover, cut-out, and a plastic piece.
Connected the dvd drive back in the 360 with the top cover off.
Put a game or movie in the drive, don't hit the eject botton, just put the disc in the tray.
Now with the disc in the tray, put the plastic piece in the center of the disc. Its magnetic, so it should kind of lock in to place.
Connect the power/av cords in the 360 and turn her on.
You should see the disc try to spin. (It may spin once or a couple times around, but it will stop). When you see the disc start to spin, GENTLE, spin/turn the disc in the direction in wants to go. It should load the game or dvd with no problems.
Reassemble the cover and put back on the dvd drive.
Now use the eject botton, and try games and dvds
If disc fails to spin, do process over again.
I've done this with one of my 360's with no problems.
The drive I used was an Hitachi 0059DJ.