I have a late 08 MBP that was doing the same thing. I installed a new battery and went from 2G to 8G of memory. This was in prep for install of a my new Snow Leopard disk I just got in the mail. Once I cracked the case I got out the vacuum and cleaned it up. This MBP is 4 1/2 yrs old and it needed it. I was surprised how much it needed it. After all the mods/cleaning I put the new OS X disk in the drive and nothing. But the ram made a huge difference.
I perused the communities looking for what other folks did about their dead drives. I read about PRAM and cleaning kits and the most 'default' answer, "Time to replace." Well, having just been inside the case to install the ram, I might have 'undocked' something. I wanted to check it out before doing anything else.
I cracked the case once again. The little ribbon that conncects the optical drive to the the MBP is rather flimsy. It wouldn't take much to dislodge it. Certainly the vacuum job I did could have disconnected it.
I took the drive out. Took the cover off it. Made sure everything was blown out (compressed air) and wiped down. I was surprised how much dirt was on the 'eye'.
Put it back together, shoved the Snow Leopard disk in and boom, it worked. This is after two days of thinking/worrying about how I was going to get my new OS installed.