Could be that it's just an old drive. That used to happen on my PM whenever I tried to insert any newer DVDs not more than a few years after I had it.
That was around the first time Apple included DVD drives so I wasn't too surprised. The one thing it could never read were burned DVDs, and eventually had problems with retail movies and the like. I finally had to put it out to pasture.
Basically, it probably doesn't like that disk, or otherwise it did before and doesn't now because it's old and cranky like my grandpa. (or, the more obvious, maybe you don't have a DVD drive?)