Some optical drives are very touchy, especially with dual-layer DVDs like the Snow Leopard installer. I'd take a look at the underside of the DVD and see if there are any scratches or smudges...if there are smudges, clean them off with a damp soft cloth. In severe cases, the drive may just not be able to read the DVD, in which case you may need to obtain a DVD that is free of scratches, or use an external optical drive, or the drive of another Mac in target mode.
Also, just curious, you mention you partitioned the drive...did you format the entire drive as a single volume? If you could detail the process and options you chose just so we can verify you didn't pick anything non-standard, that would be good to rule out.