If someone uses a decrypting app (eg. AnyDVD or DVD Decrypter) to decrypt a Netflix dvd to their computer hard drive, is it possible for netflix to find out when they get the dvd back.
You aren't decrypting the physical DVD - You are only decrypting the files from the DVD as it is saved to your computer. No, they have no way of knowing because you aren't changing anything at all on the DVD physically.