I'm having the same problem. I have an iMac late 2010 and a Macbook Pro early 2011. Both were running perfectly well on Snow Leopard, doing everything they should do, without problems. I've upgraded to Lion on the iMac and now it will no longer read original DVD films, bought or hired. The Macbook Pro still reads them perfectly well (as the iMac did with exactly the same films prior to the upgrade), but the iMac is no longer recognising them. It just spits them out before mounting them.
I've been a long term Mac user (some 15+ years) and have encountered a couple of problems with OS upgrades, but not many, but this one seems to be riddled with problems and is very disappointing. It does remind me of the frustrations I used to have when I worked on PCs in the past. It is just unacceptable that Apple don't seem to be responding at all to this problem. It is a pretty major problem for someone whose work involves burning film media onto dvds. Once they are burned, they are not recognised. But the problem also occurs with original film dvds bought from the High Street.
I am also going to report this to https://bugreport.apple.com/ as everyone should to make Apple more aware of this problem. It's a bit like upgrading to a new model of the car you've owned for 15 years and finding that reverse gear no longer works. Yeah you can get around it in one way or another, but it's pretty bleeding inconvenient.
Apple certainly need to address this problem (and all the other problems & glitches with Lion)... certainly before they even consider releasing a new OS! If they stray off the well considered path they have been on for so many years, they very quickly will undo all that they have achieved.
Sort this out!