My 2 year old PS3 Slim is in a similar situation. It stopped reading Blu Ray disks (PS3 games and movies) intermittently, but reads DVDs with no problem. I've deduced that it's the Blu Ray lens that needs a good swabbing. If that doesn't work, I may have to have the lens or entire optical drive replaced. I've also heard that there's a chance that running a "Restore File System" from the PS3s Recovery Mode may fix this problem as well. I'm a bit hesitant to try this however, because it's on kmeaw's CFW. Although I've read blogs and tuts on this matter that it shouldn't matter what FW it's on (because it's restoring files in the boot process before the FW launches) I'd still like to hear from anyone that's had to run a "Restore File System" on a CFW'd PS3 for whatever reason.
EDIT: I went to run my PS3 into Recovery Mode to run the aforementioned "Restore File System" option, but it's working just fine now. I've since decided to obtain a PS3 Lens Cleaner Disk just in case the problem resurfaces. In the meantime, I've also come across an interesting "workaround" that involves: 1) inserting a PS3 game disk, 2) waiting for the disk to not register on the XMB, 3) launching a game title from the Game option, 4) exiting the game to the XMB, and 5) *poof* the disk magically appears on the XMB. I'll keep you posted.