Had the same problem with mine about 6 months after purchase. I called the Sony help desk (number on website) and told them what was going on. After a brief chat about what I could do to fix it (system re-boot, new software upgrade etc) they took my address and serial number from the back of the console and couriered a brand spanking new one to me the next day!
The reason for the break down? Leaving discs in the machine when turning the power off!
Edit: Didn't ask how long i'd had it or anything about the warranty
Just had this with mine. Apparently it may be a bad batch of disc readers, was with my one anyway. Rung Sony and the had it picked up on the 30th Dec, sorted and back to me yesterday. Now working fine, they even cleaned it for me.
Happened to me over a year ago, sony swoped it within 2 days. My mates just had same problem and been told 19th Jan i think he said. Apparently its a problem sony know about and will exchange no problem. Do a google search its quite common.