It's not the dvd or the dvd player - it's the tv. Digital tvs do alot of processing to the signal, like scaling the input to its native resolution, applying any noise reduction algorithms, etc. That processing takes time and delays the video. That's why newer mid-high end receivers include an audio delay (aka lip-sync) feature.
You could try turning off any special settings like noise reduction or 'image enhancement' or whatever it is that your plasma provides.