The articles/posts I've seen lean towards 2 things: 1. The laser is dodgy/losing power, reading CDs uses a different wavelength to reading DVDs. Normally I'd say that'd be if its a few years old though. 2. The lens is dirty. Cleaning things from Walmart for $10 or so seemed a popular answer.
Depending on how geeky you want to be, you could rule out a hardware problem (maybe) by creating a boot CD that has a DVD player on it. If the boot CD can use your DVD drive, then you know that the issue is a software one and is either your OS or your Player software.
You could try this one Geexbox (geexbox.org) or look for a different one.
Geez, I'm afraid to open up the Dashboard because it looks like it every running widget ends up permanently taking up RAM I'd rather use for something else. It's rumoured that there'll be a Widget Manager in the 10.4.2 update (appleinsider.com)